I was actually doing a research project on average ages, so I can confirm that the year-end top 10 peaked in 2015 at 29.6 years old, after rising or staying the same every year from 2008 to 2015, then went down again in 2016, at 28.2.
It was pretty stable from about 1980 to 2009, ranging between 23.3 and 25.4, then got higher and higher after 2009. From the early years of the Open Era to 1980 saw it get younger and younger, with the first ATP years being quite hold: 1973 was 28.8. If we had ATP rankings for the late 60s, it would be even older, with 1969 probably averaging over 30, with players like Laver (31), Rosewall (34), Emerson (32), and Gonzales (41) all in the top 10.