I just feel so young..

I'm 18 and my SO is 20! We've been together for almost 4 years now and I just hate the feeling of being too young. In my province I'm not old enough to buy cigarettes or alcohol till I'm 19! My SO can buy cigarettes and alcohol and if he wanted he's capable of starting a family because he's 'age appropriate' and I'm not. This feeling sucks! I feel like I'm just 18,I'm a child but I'm ready for so much more than that! I matured early more than people my age and was always hanging with my sister and her friends.. I know what I want for my future and especially who I want it with.. It's just depressing really and my SO agrees that he's age appropriate for a family and I'm not but he doesn't want it yet anyways! I just feel so sad.. Makes me feel like I'm less and he's way above me.