So your friend is 28 in 2004 and he wants to know how old he was in 1988? (Just to clarify)
How many years has passed between 1988 and 2004? 2004 - 1988 = 16
So it was 16 years ago that the picture was taken. So, how old was he 16 years ago?
It's the year 2004 and you're looking through some old photo albums with a friend who is 28 years old at the time. You come across a photo of them when they were on holiday in 1988 having a really great time at Disney Land. Your friend says they remember it well but can't remember how old they were when they went. At what age/ages could your friend of been when the photo was taken?
What do you think the answer is btw? I have a feeling you've gone about it the wrong way above. It's somewhat of a trick question.
Is this what the question is asking: Your friend is 28 in 2004. How old was he in 1988?
Kind of ambiguous since you refer to him being 2004 'at the time' (2004 or in the albums) and you refer to 'them' but do not mention who 'they' are.
Anyway, if he's 28 in 2004 than 16 years ago he is either 11 or 12 years old depending on his birthday? Or it's a trick question in that you said he's 28 in the albums hoping the reader wouldn't catch it and that would be the answer ...