I remember when I turned 17 I was very sad and I said to my mum, "I am so old now I can't even sing the 'I am 16 going on 17' song!" and she said something like, "You still can and you can pretend you are as old as you want to be. I am sure that actress was in her early 20s when she herself was singing that song!" ^-^ WOW! My mum actually said something nice and uplifting for once! So HAPPY BIRTHDAY and don't buy into this nonsense about youth and age and "the wrong side of 30" it's all just meant to make people feel bad for getting old (something that you cannot help!!!) and buy into products and services that are supposed to keep you/make you feel younger!
20 is also still very young and adults still think you are an inexperienced idiot.