I turned eighteen a little over three years ago now, seems like a lot longer though.
I found though that I didn't need to behave any differently once i'd turned 18 it made little difference to me. Yes, there are oa lot of folks who expect certain things of an adult, but sod 'em. One doesn' have to go far to see large groups of 'normal' adults in a pub, getting considerably drunk and subsequently wetting themselves (see: My housemates when they stumbled in last night, and the ominous puddle I found in the lounge this morning.). These people are aged between 19 and 24, makes you think eh?
Personally, if the above is what constitutes being a 'normal' adult, then i'd happly settle as I am.
I wouldn;t worry about it either. If I managed then anybody can, and all that. You'll be fine. 
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Yes there are two paths you can go by, but in the long run, there's still time to change the road you're on.