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average working age?
Like, who is coding the apps that I use? Teenagers? Senior citizens? Mothers in a mid-life crisis?
With open source it's either someone incredibly dedicated to doing things for other people (unicorns), someone being paid by a company to do it (workhorses. Some might have a horn, it's hard to tell. Or the company's the unicorn), or it's someone with programming knowledge who also needs and wants to use the software they're writing (hobbyists).
Outside of the horse analogues, you probably need to look at the demographics of the users of said software and put the programmer somewhere within that bell curve. As to precisely where, I'd guess not at the low end as they've had to gain at least some programming experience along with the knowledge of the topic the software is about.
For the unicorns and the paid devs, well, they could be anyone.
There are bound to be systemic skews not accounted for here. More men tend to go into programming than women, for example, or at least that used to be the case.