Welcome ot the forum.
I can answer a few of your questions. First, 18 is not too old to start training. You're still easily young enough to get into the sport. Second, BJJ, Muay thai, boxing, wrestling - these are all very solid skills to train in. The best base anyone can have in MMA is wrestling, it compliments BJJ and lets you keep the fight where you want it.
If you want more in-depth coverage of your question, you can post it in the MMA Technique section, which you can find on the home page.
If you have any questions, feel free to ask the staff.
"You lack the requisite spine and testicular fortitude to study under me"