Do you want a large family? I wish I had started earlier because I'd love to have 4+ children. Although, I also wish I'd focused on buying a house when I was younger and before having children to be in a better financial position to have a large family.
Also, sometimes it's hard to tell how a relationship will stand up to having children. It's such a huge responsibility and change to your lifestyle (total lack of freedom, lack of free time, lack of sleep, less money due to cost of childcare etc.) that is impossible to comprehend beforehand and it takes a huge amount of maturity. It's possible when you are so young that one of you will end up feeling like it was too soon to lose all that freedom. How do you feel about being a single parent? Would you be able to continue your career as a single parent?
For some people it takes years to get pregnant, so if you know you definitely want children I wouldn't leave it until 30 or after.
For me the ideal would be to spend early twenties travelling and working intensely on your career, get married and buy a house around age 25 and immediately start trying for first baby.
I did: met exH at 18, traveling and working freelance up to age 25, panicked about my career at 25 (did an internship for no money then got super lucky and landed a job paying 100K abroad), panicked about not having children at 26 so started trying for a baby, got pregnant at 27 and gave birth to DC1 at age 28, got married at 29, also got divorced at 29, met H at 30, got remarried at 32 and had DC2 at age 33.
I don't regret: traveling, having first child before 30, working intensely at my career because it now means I don't have to work full-time and have DC2 in nursery (which was stressful with DC1), or getting married before having DC2
I do regret: not buying a house before having children, making sure exH was the right person to have children with (divorce when there is a child involved is absolutely horrible)