In our day no-one celebrated an 18th birthday. The big one was 21. I was married at 20 so never had a 21st party. I don't like to be the centre of attention so never had a 30th, 40th, 50th, 60th or 65th birthday celebration.
My 70th birthday was celebrated with a meal at home (which I cooked) with OH, children, grandchildren, and (surprise guests) my Mum and brother.
For my Mum's eightieth we had a "girls weekend" at Centre Parcs with Mum, her two daughters and five grand-daughters. It was great fun because Mum was very fit and sporty at that age. If I get to 80 in such good nick I wouldn't mind another "girls weekend".