I was 19 with my first, 23 with my second and 28 with my third. I am glad i started out young because i always wanted 3 kids and ideally to have them all by 30.
I think 19 was pretty young and i was not nearly as prepared as i could have been however, i also think that you are never truly prepared to have kids no matter what age you are. I feel like i was much more patient with my second and third and that my son suffered a bit from being the first born (i didn't breastfeed him because i was too overwhelmed at first, i didn't understand the importance of a schedule so to this day he is not very organized or structured which affects him in school).
On the other hand i am more "real" or i guess more myself with my son than i am with my other two, partly because he is older but also because as a younger mom i treated him differently (i guess more laid back, more of a sense of humor) than i do with second two. It's hard to explain what i mean. I think he sees me as person more than my other two, not just as his mom, because of my parenting style that i had when i was younger that i lost as i got older. For example, he calls me dude, we joke around more like peers in a lot of ways. One time my daughter's homework said "ass Assignment" at the top of the page because the CL from class didn't get xeroxed onto the page. He pointed it out to me and we laughed for long time about it. I don't really have that same kind of relationship with my younger two kids.