TLK

I Generation: King Mflame & his family
II Generation: King Dhahabu & his family
III Generation: King Mohatu & his family
IV Generation: King Ahadi & his family
V Generation: King Mufasa & his family
VI Generation: King Scar & his family
VII Generation: King Simba & his family
VIII Generation: King Kovu & his family
IX Generation: KIng Kifungua & his family

cub Simba and Nala: 4 months
adult Simba and Nala (TLK): 4 years
adult Simba and Nala (as of Kiara's birth): 4 years, 4 months***
Mufasa and Scar (at time of stampede)*: approx. 5 years**
adolescent Nuka: 18 months
cub Kovu and Vitani: 8 months
cub Kiara: 3 months
adult Nuka: 4 years, 3 months
adult Kovu and Vitani: 3 years, 5 months
adult Kiara: 3 years

*I tend to group Sarabi and Sarafina in this age category as well, though there's no direct evidence to claim that all of them were cubs at the same time. Likewise, I presume that Mufasa and Scar are littermates, though using just TLK as a base allows for some flexibility on the matter. Zira I personally think of as a year younger than the brothers, though, again, there's no evidence to directly support it nor rule out her being the same age (being older is doubtful) or even younger.
**Assigning a specific age here (year, month) would be largely speculation, as there's nothing to provide so precise a deduction. All other ages are considered fairly precise, though there could be a month or two leeway to either side for adult Simba and Nala (TLK).
***Presuming Kiara was conceived in CYFtLT or shortly after Simba's return and that Simba and Nala were 4 years, 0 months at the time (no leeway as mentioned above). The gestation period for a lion is 110 days, btw, or roughly four months.

Per a reference in The Art of TLK (explaining the illusion that, prior to the characters speaking, we are led to believe that these are essentially real lions), I've always assumed that the characters aged in concordence with how their respective species actually age. If another time scale is used, then the ages arrived at above are a moot point and largely academic.

Not being anything even approaching an expert on hornbills, I really couldn't say. However, by using TLK6NA as a reference point, it would be possible, I think, to get at least an approximate figure for Zazu's age as of Simba's cubhood. That said, I'll admit I'm quite lacking in the way of books on the subject. Thus, in an attempt to answer the question in some form, I resorted to internet research (I much prefer books and journals, what with peer review and all). Here's more or less what I've found:

* By all indications, Zazu is a red-billed hornbill (Tockus erythrorhynchus). Not only do the beaks match up, his appearance in concept artwork is very indicative of the species. That and an article by the Oakland, CA Zoo claimed in as much.
* The longevity of the species seems to be undecided, since one site said that they've lived to eighteen years in captivity and another said that they've been known to live past twenty (unclear if in relation to captive or wild bird). Still another said it was unknown how long they lived in the wild.
* Young remain sealed in the nest (check out hornbill nesting behavior, it's wild) until they are roughly two months old.
* Sexual maturity is reached between one and six years of age (size seems to be determining factor). No site said when the young become independent, but I would assume it must be by sexual maturity, barring occurances of delayed dispersal, which I saw no evidence of. Also, breeding seems to be a yearly occurance, so offspring are probably gone by the time of next breeding season.

Now then, applying this to Zazu:

In TLK6NA (specifically book one, "A Tale of Two Brothers"), Zazu's mother, Zuzu, describes going on "exercise flights" with her family, which I presume includes her offspring. If such is the case, then by the time of the story, he's obviously past two months of age. However, if he's still with his parents, then he's also under a year of age. For no reason beyond the sake of easy computation, let's say he's six months old at this point.

Here, of course, is where Disney starts bending the rules, as it were. In "A Tale of Two Brothers," Mufasa and Scar appear to be about two-and-a-half years old on the basis of degree of mane growth (with the exception of Nuka and that quick view of adolescent Simba, Disney has failed miserably in depicting the pattern of mane growth). By that same margin, in a later story (book six "How True, Zazu?") taking place early in Mufasa's reign, Mufasa (and Scar, though he's not shown) appears to probably be around four years old.

Now, how does this reflect on Zazu? Simple, Zuzu retired from the position of majordomo right around the same time as Mufasa's ascention to the throne (presumably to travel). Zazu, obviously, took her place. The implication seems to be that he was still living with her some one-and-a-half years later, which, if I'm inferring correctly, would be...odd for a hornbill. However, hornbills are monogamous and there's no mention made in any of the stories of Zazu's father, so it's possible that Zuzu was a widow and so there was less need to get last year's clutch out on its own to make way for a new brood.

By the descriptions of Zazu in book six, he's probably not quite an adult or is a very young adult by that time. Obviously, come TLK itself (approximately a year later, if I've got the lions right), he's matured.

Thus, on the basis of scanty information for a starting point determinant, we're left with this hypothetical age chart for Zazu (all ages approximate, even those with months):

stampede: 3 years
TLK finale: 7 years
SP act I: 7 years, 7 months
SP act II: 10 years, 7 months

Presuming that red-billed hornbills can live into their teens in the wild, Zazu's still got a few more years in him before he can officially be deemed a senior citizen.

Of course, as mentioned, this is all based on rather incomplete data. There is at the campus library a considerable volume just on hornbills. Since I've got some books that I need to return anyway, I might just take a peek at it and see what it says about the aging process of red-billed hornbills.
That is a good deal harder, since Rafiki's species is open for question. Mandrills have the colorful face, but they lack a tail, live in rainforests, and have a range that doesn't overlap with that of lions. Savanna baboons do have an overlapping range (they live on savannas, as their name implies), have a tail, but lack the colorful face. Yeah, so, unless the aging process is the same for both species, it'd be hard to piece together an approximate age for Rafiki.

I should note, I was thumbing through George Schaller's The Serengeti Lion (a classic piece of leonine scientific literature) and have since revised my approximation on Mufasa and Scar's ages. Here's an amended and extended list for the leonines:

Mohatu (The Brightest Star): at least 6 years, no more than 8 years
Ahadi (TLK6NA, "A Tale of Two Brothers"): at least 7 years, 6 months, but no more than 8 years, 6 months
Mufasa and Scar (TLK6NA, "A Tale of Two Brothers"): 2 years, 6 months
Mufasa and Scar (TLK6NA, "How True, Zazu?"): at least 4 years, no more than 5 years
Mufasa and Scar (at time of stampede): approx. 6 years
cub Simba and Nala: 4 months
adult Simba and Nala (TLK): 4 years
adult Simba and Nala (as of Kiara's birth): 4 years, 4 months
adolescent Nuka: 18 months
cub Kovu and Vitani: 8 months
cub Kiara: 3 months
adult Nuka: 4 years, 3 months
adult Kovu and Vitani: 3 years, 5 months
adult Kiara: 3 years

Young Simba: Age six

Young Nala: Age six

Adult Simba: 18

Adult Nala: 18

Scar: Before taking over: 30-35, after Simba comes back: 48-53

Mufasa: 30-35

Sarabi: 28-33

Sarafina: 25-30

Kiara: Young Kiara: 5, Adult Kiata: 17

Kovu: Young Kovu: 6, Adult Kovu: 18

Vitani: Young Vitani: 7, Adult Vitani: 19.