I believe that even into the 50s or 60s it is still very much possible to achieve a sub-15 average. It's possible that older people improve slower, but I think it's more likely that there are other factors holding them back, such as:
- lack of time to practice, compared to teenagers
- most of your life spent not moving your fingers quickly
- psychological effects - thinking you are too old/slow to have any chance at being fast
- unwillingness to push yourself and go faster than you are comfortable with
- lower neuroplasticity, making it harder to learn completely new techniques such as lookahead
My evidence? Look at professional pianists. Many of them play extremely complex and difficult pieces, which require finger dexterity and speed that is comparable to sub-15 averages (if not faster ones). However, age does not affect pianists nearly as much as we think it affects cubers. Being able to play properly (i.e. continue their career) until middle or old age is the norm, and indeed, many professionals seem to improve with age, as they get more experience and learn more pieces. Youtube has tons of examples of this.
Oh, and for what it's worth, Ron van Bruchem got a 12.62 official average when he was 44.