I don't have the data for the average age, but I was recently in a conversation around how long does it take to become a committer since getting involved in Postgres by writing code for it.

So, I wrote a couple git commands like below [1] to figure out when someone was first named in a commit message vs when they made their first commit (as a committer) for the last 10 people who became committers.

The average time of involvement was ~8.9 years (just comparing month / year), with the lowest being ~6.5 years.

Obviously one could do better analysis but my goal was just to get an approximate understanding.

[1] git log --grep 'Name' --format=%cs | sort | head -1

git log --author 'Name' --format=%cs | sort | head -1