Cron,
As your profile shows your location in the UK, I assume you are referring to JAR-FCL regs?
If so, there are no age limits to instructor ratings. The only age limits in FCL refers to Commercial Air Transport, something instructing is NOT.
There might be additional, national regulations, though. I am no expert on UK national reg's, but I am sure others here can inform you.
Cheers,
Redbar1