I was 18 when I had DS (10 in July), and I know exactly how the OP feels! (sorry, this is going to be long!)
When I was pg, I would get the tut tuts from old fuckers who thought I was still at school, even when I had my work clothes on!!!
The midwifes and staff at the hospital (except one old shit who worked the night shift), were actually realy nice to me, which surprised me, it was the other mothers who looked at me and spoke to me like shit! I was on my own cause DP was abit of a prat back then (we got back together when DS was older though), and they all had DH's and the woman in the bed across from me would talk loudly about me to her visitors, and they would join in with the comments and dirty looks!
On our first outing, I was on the bus and this group of old bags and a man (I find old men are worse than old wifies!) were banging on about 'school girl mums, scrounging off the state', I hadn't even looked in their direction, they had no right to make those judgments about me. Luckily my mother was with me, and she gave them what for!!! She made me get my pay slips out of my bag, and made it well known to that old shits (and the rest of the bus!) that I WORKED and paid my own way!
Although I did end up on benefits after my mat leave ended, but I was not sitting on my arse all day doing nothing, I was at college getting my highers, then after I left, I got a job and have worked ever since.
Someone else in this thread has mentioned over-zelous security guards, I got that too! It got so bad, that I only felt comfortable doing my shopping whilst wearing my work uniform! They never seemed to bother me when I was in work clothes!
When DS started nursery, I was the youngest mother, most of the other mothers were in their late thirties/early forties! Guess what, they also looked down their big stuck up noses at me! If any of their DC were naughty in the park, no big deal was made of it, however if DS did anything they would make a big deal of it!
However, theres lots of good things about being a young mum, I had tons of energy back then, and when DS grows up, I'll only be mid-thirties, so I'll still have my life in front of me.
Just hold your head high, you have nothing to be ashamed off, and as for some posters saying you sound immature, dont worry, I still feel and sometimes act about 16!!! LOL!