This thread might give you a couple of clues. As you'll gather, pay for non-airline professional flying ranges from pretty poor to below-minimum-wage levels. That's not how I'd recommend anyone invest £100,000 at your stage of life. If the money is just something you'd fritter away anyway then flying's as good a way to fritter it as any, but if you seriously need to earn a living, you want to spend as little of that capital as possible.
If you read
this thread, you'll find all you need to know - including how much it'll cost you if you're careful with your money. Within that thread you'll find a link to the 'Am I too Old...' perpetual thread. To cut it short, you're not - but it's not easy as you approach 40.
Good luck!
Scroggs