I would train both at the same time, i do. have done since i was 16 years old, i had plenty of Gracie Jiu Jitsu experience before that.

Train both together.

I've been training muay thai for about a year now with a little bit of bjj and i want to transition in mma ,so im good at muay thai but i live in stockton so obviously the cesar gracie academy (nick and nate diaz's school) is right next to me.my question is continue with muay thai or do bjj??? i will end up doing both, but what should i do now? Im 16 and want to get mma fights at 18.. Thanks :)