In terms of the trade, I started to get into plumbing age 28, I think.
But in terms of plumbing, I'd been DIYing stuff since a relatively early age. I definitely changed a part 2 float valve diaphagm 'washer' age 14, and it went from there, though that might not have been the first job I undertook and most likely wasn't.
I first considered the trade might interest me when, at 24, I had had three separate RGIs look at my own Potterton Profile boiler and none of them could explain why the boiler occasionally boiled and tripped out, nor why it was consistently running 10 degrees above setpoint. None of them actually investigated. I eventually got fed up with the so-called professionals in my area and did my own investigation in order to realise the boiler had been plumbed in backwards (for years) and was, thus, sensing the return and not the flow temperature. Funnily enough, I've never actually gone down the gas route and I'm still in two minds as to whether to bother or not.