Is it normal to find underage girls attractive?

First off, I'll clarify that I'm not in any way condoning any sexual or inappropriate relationships with minors. Would never do such a thing.

In my country (the UK) the age of consent is 16 - no Romeo and Juliette laws here or anything like that. 15 is illegal, 16 isn't.
Anyway, so you know how when you're out and about you see people and sometimes you thing "Wow, she's hot."? Well quite often these are just as likely to be 13, 14, 15 year old girls as they are girls my age (I'm 23). I'm not talking necessarily about girls that look much older - but girls who have already pretty much gone through puberty so they have a female figure (not talking about girls who look like young kids).
Now obviously this is not something you can openly talk about because it's so frowned upon in society. But males I know quite well seem to drop hints that they're the same - for example, I was at work the other day and my colleague in the van next to me was looking at this group of girls who can't have been much older than 13. I looked at him and he said (jokingly) "If they were a bit older...".

Is this perhaps more of a common thing than we think?