I'm 24, I started when I was 22.
I can't say if guys using prostitutes in their late teens and early 20's is unusual as I don't have a full breakdown of punter demographics.
What is interesting is the opening up of the market with adultwork. Those a few years younger than myself are known as digital natives. I.e they have never known life without the internet. If you wanted you could make the slightly long leap which says because access to prostitution is now easier with a wider market, that more young guys who think about casual paid sex will go for it.
The second question to ask is (and i'm genuinely not trying to chat BS) do punters have a certain mentality that makes them punters? For me personally my experience has been shaped through the lense of disability. When I was 18, I started to read articles on sex and disability. you'll all know the sort of thing, should disabled people have a right to sex? Would you have sex with a disabled person? etc...the conclusion I came to was that although there is always a chance for a connection and a meaningful relationship I wouldn't feel like a man until i'd lost my virginity, there was no chance of this happening with my lack of knowledge about my own body, with my lack of self confidence and with the way in which certain people view disability.
My solution after much soul searching was seeing escorts. A few years have passed, fluffiness has turned to awareness, caution, and a knowledge of what I want out of a punt, Would the 22 year old me have done all this without the internet?
Hell no. No in depth punting knowledge, no academic or scholarly articles to weigh against the moral arguments, no way of knowing who you are going to meet. Add to this the fact that no prostitute has ever wanted a "long" phone call. how would I have ever had a chance to explain my disability? A good proportion of escorts are used to seeing guys with disabilities these days, thanks to sites like the TLC trust, and documentaries from our beloved channel 4.
To get back on point I can't say if it's usual or unusual, but given the ease of setting up a meet versus the desire to explore sexual fantasies it may be more common than it was.
HMD