Puzzled by the wording

I bought a card for my grandsons 16th birthday. It said ‘Born in 2002 now 16’ or something similar. The problem though was the year. When he opened the card, grandson chortled and said ‘But I was born in 2003’
The fact of the wrong year had not clicked with me in the shop because I thought it must be right.
After a lot of head scratching I went back and had a word with the man who sold it to me. I asked how it was possible to celebrate your 16th birthday in 2019 if you were born in 2002.
‘Well’ he said, ‘I had it all explained to me by the supplier. it all depends what month you were born in and how far the year has advanced.’
Huh? I need time to process a claim like that, so I kept it good humoured, went home and considered the conundrum.
Of course I now realise the man was talking tripe, unless he was as daft as me and taken in by the supplier’s lies, but somehow I doubt it.
If you are just 16, you must have been born in 2003...
Obviously...
Right?