Considering I'm in my late 20s, I'm sure the number will rise.
For now when I was 17 I was in Europe on vacation and got lucky with my tour guide who was as I recall 28 or 29.
It basically went like this:
I was in Germany and wanted to go on a small tour of Italy, booked with a tour company for 8 days, went from Frankfurt all the way down to Naples. The funny thing about the trip was that most of the other tourists were retirees. There were like 70 year old couples, etc.
I was the youngest by a country mile to a 33 year old who seemed to be nearing 40.
In short, I spent a lot of time talking with her as she ate alone from the tourists and I would eventually make it common to eat with her, the last night we all went to this beer hall/dance hall and she told me she was an extremely light drinker, she didn't drink too much before she did start to seem more "fun". I then, as drunk as I was, asked her what she thought of me, she said something along the lines of me needing longer hair. Late that night, in a drunken haze I got invited by her to her hotel room and I ended up having some very boring drunk sex. When I woke up the next day at around 10a.m. she had long showered and was down stairs in the lounge drinking coffee, she seemed to be completely fine with what had occurred between us and after a bit of chit chat I simply took a train to Munich and then to Hungary.
Sorry, I always have to tell this story as a decade later I cannot believe it actually happened, seems something out of euro trip.
So the answer is 11/12.