Even though there was no mention of sexual activity, the age of consent in UK, Canada and many US states is 16, so even that would seem legal (though not advisable) in most cases. It's interesting to contrast the response to recent question if 9 years is too much difference when the other person was 20... Though, the "rule of thumb" of half your age plus 7 suggests that 24 years should stick to 19 or older to keep it non-creepy. People making 18 a sticking point is also bit... ehh.
People mature at different rates, but 18 has been given special significance as the "average" because, well, let's face it everybody wants to believe they're the special case. What I mean to say in particular though is, not every 18 year old is ready to date whatever age they want. In fact, I would say that up until they've finished whatever education they're going for (Under no duress), people shouldn't date way out of their age- or life-situation category for variety of reasons. I realize in the real life this is little idealistic; however, to answer the question, in general terms "age is just a number" only once both parties have finished their desired education.
