It's not like your body just stops growing as soon as you turn 18 at 12:00AM. It does slow down, but getting the right amount of nutrition, sleep and other contributing factors to growth will help you grow a little more. I wouldn't worry about your height that much, there's so much more that you could be worrying about man.
Puberty is also a huge factor, once you reach puberty you grow a shit ton. Women stop developing before men, so if you're a dude – you may grow a bit more!
When my cousin was 18, he was 5'10, but then by 20 he was 6'2. So it's hard to say when you fully stop growing, everyone is different