There is no age minimum for tithing.
Many children at Mass are given money
by their parents to put in the basket;
and when people are old enough to
earn their own money, they may give
according to their means.
If, by chance, you are interpreting
the word tithe literally to
mean giving ten percent of your income,
the Church has never required this.
Instead, She emphasizes the spirit
of this number to mean whatever substantial
gift, or percentage, a person, in
good conscience, can appropriately
afford (and/or) feels inspired to