Will a child passport last for the full 5 years?

Question

if i buy it for a 15 year old

Answer 1

he will probably have to get a new one when he turns 18

Answer 2

Are you asking if s/he can use it until expiration?  Yes.  

S/he will have to renew it in person.

Answer 3

Passports for children under 18 last 5 years, over 18 lasts 10 years.  If the 15 yr. old gets a 5 year passport now,  he/she will have to get a new one when theyre 20.