I'd been admitted to psychiatric hospital two weeks previously so I was still there. Saying that, it was a Saturday and as not much goes on at weekends and they considered me well enough by then they let me go home for the evening/night and come back the next day. Went out for a meal with my parents but I can't remember anything much about it as the medication messed with my memories and everything from then onwards for the next 6-8 months is rather fuzzy. I had had some sort of friends at school, but we hardly ever saw each other out of school and even though we had only left school a few weeks previously (my birthday is July) we had already lost touch pretty much permanently, so no chance of any type of celebration with them.
Most of my 21st was spent alone - it was Uni holidays so although I did have a couple of friends they were at that time in different parts of the country. As far as I remember I carefully organised my day (everyone else was at work/school) to do things I liked doing on my own - walking, picking fruit, playing my guitar. It was going to be perfect, then my Grandparents phoned wanting to take me out to lunch and as I couldn't stand the disruption of my carefully laid plans I refused the offer, and instead spend the rest of the day feeling guilty and upset that everything had been ruined. In the evening I imagine I went out for dinner with my immediate family but I can't really remember that as 19 and 20 were pretty much the same.
In general birthdays (since childhood when I loved them) have been disappointing even when I have had friends. A lot in my 20s were shared with my (now ex) husband who had his the day before mine. They were generally OK, although I remember 25 being upsetting after I invited our friends (I had quite a few at that time) to a BBQ and made loads of food and then it rained and few people came. After 30 and 31 being pretty terrible (I was depressed and had few friends at the time) I didn't tell anyone about 32 as I couldn't stand celebrating, but it was sort of sad. Last year (33) I went out after work with the lovely people in my lab and we had a couple of drinks and it was nice. I think big birthdays have the potential to be terrible if celebrated and sad if not, but luckily it will be a while before another one of those.