Because past then, chances are they'll live away from home and then it's not about age it's about being in a completely different stage of life (ie: adulthood and not being a teenager).
When I was 16 I went out with a 19 year old who was at uni and he was way too old in retrospect. But then 18 and pretty much any age I think is fine.
Depends on the maturity of both, but up to about 20/21, would be my limit.
I first went out with my bloke when I was 19 and he was 24, and we both lived at home. If we'd met up when I was 16, then he'd have been 21. No problem.
when I was 16 I met and dated a 22 year old, my mum new and she firmly believes that I need to learn my own way through the world with things like that, I know that if she had told me I couldnt date him we would have fallen out unessesarily, I believe that people need room to make their own mistakes as that is the only way they are going to listen, Im now older and happily settled with someone a similar age, dont worry about it!