It's never too late to start something new. Od Hagen didn't start competing until he was in his 40's, he's now in his 50's and still competes on the world circuit.
As far as incorporating it into a bodybuilding routine that's going to be tough. You can use many of the motions/events for training but in order to train to compete imo you will have to choose one or the other. You can stay lean by maintaining a clean diet but you have to move away from hypertrophy training and into strength training. Also many of the marquis bodyparts in bb are neglected in strongman training. Chest for instance becomes a stabilizer and not a show piece.
In order to compete you just need to find amateur competitions to start. The best piece of advice I can give you is to start competing as soon as possible. Many people want to get good and then compete which is assinine to me. You learn more in 1 competition than in months of training. Also you get to meet more experienced people who can help you tremendously. The most important thing in order to do well in competition is to train using the implements. It doesn't matter how strong you are if you don't know the techniques involved you aren't going to do well. Also as you will find out very quickly in order to compete in strongman you have to love it. It is pure torture and pain so if you don't love it you'll quit very soon after beggining. I will warn you though if you get bit by the strongman bug it becomes a true passion.
The other great thing is that there is no contest dieting. Unless you need to make weight for a competition there isn't ever a need to lose weight. At least here in the US we have a full set of weight classes at the big comps (up to 180, 180-200, 200-231, 231-265, 265-300, and SHW). Smaller comps have two weight classes light and heavy with 231 being the cuttoff. So you don't have to compete with people far bigger than you for the most part.
Cheers,
Scotsman