[SSJKarma]

first: i live in quebec province in canada !
we have 6 years at PRIMARY school and 5 years at high school then we're gone to colege !
we enter primary school at the age of 5, for another reason i entered at 6
then do the maths and you'll know what i mean !

now...
clearly stated in the manga as well !
gohan was 4 when raditz came on earth.
clearly stated in the manga that he was 5 during the vegeta and nappa fight !
clearly stated the the namek saga was about 1 or 2 years
trunks arrive and he gets another 3 years of training
then cells come in and gohan trains 1 more year !
then 7 year later buu arrives !

EDIT:
that is with the same resoning as TarkanX...
he goku had had 21 when he got gohan and then he clearly say he had 25 when fighting raditzs !
now 25-21 = 4 year old
that's the age of gohan
now lets recalc...
4
5
6
9
10
he was 10 when he fought CELL and he is 17 !
but i really think 2 year passed before trunks came in !