Moving toooo fast? 😳

Hi, my boyfriend is 21, and I'm 18 and he and I are in mad love, I know I'm young and all that crap but I love him a lot. Anyway we've only been together for 5 months and he's already talking about marrying me once I'm 21, and I've always told myself that I didn't want to be married so young, I wanted to wait till I was at least 25-26 or whatever. He talks about our future together A LOT. I think it's super cute that he's thinking about it, and yes I want to be with him for the rest of my life but, I just wanna slow down a bit... I feel like the whole marriage thing at 21 is a bit ehhh.... What should I do...?