The problem with age is that it is simply a number!
As you point out a 17 year old was apparently not very mature, but on the reverse a 15 year old may be VERY mature for their age!
The specifics of this particular case is something most here only know about through the media.
Whatever anyone feels about the footballer, we would not be debating this if she had been a little older, so what makes it fine at 16 when you yourself say 17 was immature?
Some seem to think at 15years 364 days he is a disgusting ****** who should have his balls removed, whilst a day later its fine.