We've never had any teenagers on our tours from 2016 through just last month in 2023. All have been shoulder season - no summer travel.
There was a younger (25-ish) couple on our 2016 Best of Paris tour.
Every succeeding tour seems to skew older. Our 2017-2019 tours (Italy, England...) were mainly middle-aged and senior couples with several 30-40-ish couples & singles.
Our tours this year (Belgium & Holland in late April, Berlin-Prague-Vienna in May) were composed almost entirely of seniors with half already retired and others who were still working into their 70's. There was one middle-aged aunt with her 20-something niece on the B-P-V tour.
We were surprised to see so many people older than us this year and we're retired ourselves (66 & 67). ) The first tour of 28 was reduced to 26 by covid and there were 27 on the second tour. The second tour group developed a great camaraderie very quickly which was noted by the guide. The great disparity in ages between the niece/aunt and the oldest members was hardly a factor at all.