I'm not as old as some of the examples mentioned but I started late (still can't read music) and I reckon you can't ever be too old to have a go. So long as you're playing for your own enjoyment you'll be the only critic of any performance; just try to be encouraging to the performer. With nobody to hear you can fluff all you like and you'll only get better.

Sooner or later you'll be overheard by someone close and they'll probably be impressed with your progress.

I've never played fiddle but it's a bit of a trick to sing while playing. Melodeons tend to want to breathe differently from people so singing with them can also be a trick. Tricks can be mastered after a bit of practising.

Cheers, Rowan