"How old is your uncle?"

[gpriddy]

Why can't I include "il" before "tuo zio"?

[gewisse]

When using possessives with family members the definite article is omitted in the singular ('loro' is an exception - this keeps the definite article). In the plural the definite article is used.

e.g. - mia sorella / le mie sorelle = my sister / my sisters