Learn Spanish (Latin America): Mi tío tiene veintinueve años.

In Spanish, you always use the verb tener (to have) to talk about age. Instead of saying your uncle is twenty-nine, you literally say he has twenty-nine years (tiene veintinueve años).

How do you build the word "veintinueve"?

Numbers in the twenties are written as a single word! It combines veinte (twenty) + y (and) + nueve (nine). When they merge, the end of veinte y changes to veinti-, giving you veintinueve.

Are numbers from 30 onwards also written as one word like "veintinueve"?

No, that one-word rule is mainly for the teens (16-19) and twenties (21-29). Once you reach 31, the words separate. For example, 31 is treinta y uno (thirty and one), not a single combined word.

Does "Mi" change if I am talking about multiple uncles?

Yes! The word Mi (my) must match the number of people or things you are talking about. Since tío (uncle) is singular here, you use Mi. If you were talking about multiple uncles, you would add an s to make it Mis tíos.