Just thought I’d post this info.
From Wikipedia (which we all know is NEVER wrong), John Snape was born in 1978 which would put him at 29 (which is kind of what I was figuring). Fizz was born 1985 which would put her at 22 during these episodes.
So there’s a 7 year difference between them which means that if Fizz was dating him at 15 or 16, he would have been 22-23. Risqué perhaps, but certainly not on the creep level of Martin and Katie or what is happening now with Rosie. (Although, I don’t know about the laws in the UK, would it be illegal for a 22 year old to date a 15 year old?)
Oh, on a sidenote, don’t go into Wikipedia if you don’t like spoilers. The whole Rosie/Fizz/John thaaaang is played out there.