There's a 6 month time limit on the 'laying of information' for 'summary' offences (which can only be dealt with by magistrates' courts) but there's no time limit for 'indictable' offences which can be (or sometimes must be) dealt with by a Crown court.
The Crown Prosecution Service would have to consider two important factors before bring a case:
Firstly they'd need to consider whether they could PROVE (beyond reasonable doubt) that unlawful sex had taken place. Even if the man had openly bragged about it he could then say that it was simply a young man's bravado, and that nothing had actually happened.
Secondly, they'd need to consider whether such a case was 'in the public interest'. They might well decide that there would be nothing to be gained by launching a prosecution. (If it would cost the tax payer a great deal of money, risk destroying family relationship and have no guarantee of success, they might decide against taking any action).
I've taught in a secondary school, for 15 years, where it was widely known that many (possibly most) of the 14 and 15 year old girls were having sex with (usually slightly older) teenage lads. During all of that period I never heard of a single case ending up before the courts.
Chris