I don't think that applies to tennis. I backed a player last year who retired after the first set - turns out he was sick the night before and probably wasn't going to turn up. Add those instances up over 8 years including non-triers, guys who wanted to leave ASAP to get to play the qualifying rounds of the next event and it could easily mean the difference between a positive or negative P&L.
I started at around your age and I had some beginners luck before I had a few tough losses which I learned from. I used to have a go on the flat racing but I find it a minefield now. Jockeys taking a pull around the outside of the track when there was no need to, usually following a big drift in the Betfair market...
That sort of inside information is hard to come by as it will probably be kept between sportsman and trainer and neither will want to say anything.
i cant wait for the flat season to start again i find winners so much more easy to predict.