My SO maybe be scared about marriage😕

I'm 21 I've been dating my bf since I was 17. Great relationship, both live separate on our own, however he's noticed I've been looking at wedding rings and even made a wedding page on Pinterest (I know I'm not the only one lol) but I've been bringing up weddings, marriage and babies a lot.. We've talked about it numerous times and he's sure that I'm the one he wants to marry, but lately he's been acting strange when I bring up marriage he keeps saying we're too young but I feel like I should at least be engaged right now 😕