I took early retirement from my desk job in industry at age 60, by which time I'd spent about 3 years doing evening classes (on the quiet) to get my NVQ2 and then finally my NVQ3. I then immediately became a sole trader and am still enjoying my 1-man-band plumbing career in the 'evening of my life' - as they say!
It's been 7 years now (I'm 68 later this year) and I'll keep going as long as it's still fun and I can still double up under a kitchen sink. Money for old rope as far as I'm concerned, and you get to meet a wide cross-section of people - some of them really nice.
So I suppose I'm answering your question with a 'yes' - go for it! The only thing to remember is that initially the money's not going to be that good, because it takes a while to build up a decent customer base - and also your plumbing skills and tool collection. Eventually you hardly need to advertise - you get a lot of jobs via referrals and recommendations.
In my case I had a small pension to help out plus some wifely income, and also could live quite cheaply (kids moved on), but I do realise this may not apply to you.