Why тебе? And not тебя?
Indicating age is done in dative case (lit. “you are given 5 years”), so you use тебе which is dative, unlike тебя, which is both accusative and genitive.
Года vs лет?
"Года" is genitive singular, "лет" is the irregular genitive plural. Genitive plural is used with numbers ending 5 and up. Genitive singular is used with numbers ending 2-4, except 12-14 which are also genitive plural.
This is my grandma when she hasnt seen me for a while