Dirty Old Man?

Today at work, once again, I felt old.
We have a group of guys in the building working on the sprinkler system and last week 2 new younger guys joined the existing group. Both are pleasant looking lads around the age of 18-20 and I found myself thinking about one of them, mainly because he shows off his nice arse by wearing smooth, black Adidas-type bottoms.
I'd chatted to them a couple of times before and then again this morning when they came in to work I was chatting to them in the lift, where it was just the three of us. When I got out of the lift I felt like a dirty old paedophile grooming 2 young boys. This feeling was made worse later when the receptionist saw one of the guys and commented on how young he was and from that point I couldn't help but imagine him in a school uniform (short trousers, rolled-up white shirt sleeves and a loose tie, topped off with a green school cap slightly off centre). I felt like even more of a paedophile then.
I know it wouldn't be illegal to go with one of these guys, but to me it would still feel wrong as I am old enough to be their dad!
I've always thought of it as an unwritten rule that I would never have sex with anyone born in 1988 or later. This is the year I left school at age 16 when it was legal for me to have sex (though it was still illegal for me to have sex with a guy until I was 21 at that point, not that it stopped me!).
I am going to try and keep to this rule, as I think it is a healthy one. But, if someone were to come along and change my mind I wouldn't fight it too much. I'm not going to turn away love if it finally comes along.