No idea where you'd get started, but Marland Yarde only started rugby at 15 I believe, and is now on £200k a year and when he finds some form should be in with a shot at international honours again. Nathan Hughes at Wasps is looking nailed on to be included in the England squad, and currently the best 8 in the AP, and only started rugby at 17.
It's doable, but you need to be a good athlete!