I estimate young Simba and Nala at the age of 9, 10 years
At the end of the first movie I estimate Simba 25 and Nala about the same age.
Mufasa and Sarabi are, what I think, begin 40's when Simba the cub is presented to the animals of the Pride Lands. Rafiki must be, indeed, in his 70's! Zazu must be like the same age of Mufasa, since they're like "ol' buddies" or something...
I estimate young Kiara 8, and young Kovu 10 in the 2nd movie. I estimate Vitani 11 or 12, since Zira said Kovu was the last born cub thus younger than his sister Vitani. I estimate Nuka 16 at that time, in human years.
I estimate Simba and Nala in their mid-30's in Simba's Pride.
Adult Kovu and Kiara must be like 18 or 19 years in that movie, I guess Nuka would be in his mid-20's at that time.
Strange thing is that Zazu, Rafiki, Timon and Pumbaa doesn't seem to be aging in these movies!