@Digi-Lib
After reading it, the way I see it is:
First chapter he's 8, and later on he has his 15th birthday.
This chapter he's 16, and he's mentioning that 15 years ago he celebrated his first birthday...
Because he was a year old at the time. Or at least, that's either the estimate or the date his grandparents found him (I forgot and too lazy to check a dozen volumes). And you celebrate your FIRST birthday after a year of your life passed. He's celebrating 16th now, because 16 years passed since he was adopted etc.
So yeah, he's 16 now.