Tough question…it truly is different for everyone. I had my first just after I turned 28, and now pregnant with my 2nd at 29. I’m looking at hopefully being completely done with my childbearing by 32. Having kids younger was absolutely the right decision for us because we knew we wanted to have multiple kids (most likely 3), and we were not worried about achieving FIRE or anything like that before kids arrived.
From a fertility perspective, I would recommend that you start trying relatively soon (within the next couple of years) if you want the option of having more than 2 children open to you. And most definitely start sooner rather than later if you are a lady with a history of problematic cycles or anything that might hint at anything less than normal fertility. The problem with delaying too long is not that your risk catastrophically rises in your early 30s (the really scary declines in fertility often start around 37 or so), but that your timeline is cramped if you do run into trouble. For example, if you start trying at 31 and find out that yes, you do need fertility treatments, you can still easily end up being a mom by 33. But if you start trying at 33, and need fertility treatments, you might not become a mom until 35. This won’t matter at all if you want only one kid, but if you know you really want to remain open to multiple kids, you don’t want to be limiting your timeframe from the get go unless you really need to for some reason or other. The time adds up rapidly…if the first baby doesn’t arrive on scene until age 35, and you wait the recommended 12-18 months between that birth and trying to conceive again for baby #2, and it takes you the average of 6 months to conceive that baby #2, you are looking at being pregnant again at 37-38. Again, not bad if you only want 2, and most likely will turn out fine (provided you don't run into any problems like miscarriage or fertility issues). If you want 3 kiddos however, you are looking at being a 40 something with a newborn and a very high risk of miscarriage and certain genetic conditions. Many women find that once they hit their late 38s/early 40s, the option of having another child has been taken from them.
Personal anecdote – we started trying at 26, and despite perfect reproductive health, just had a bout of bad luck and had back to back miscarriages before conceiving our daughter. It took 10 months to conceive a viable pregnancy. With this baby now, it took 6 cycles of regular timed intercourse at age 29 (average). I am very thankful we started trying early, even though everyone said we were "so young"…it was bad enough going through those losses and wondering if there was something seriously wrong with us, and I am glad I didn’t have to worry about age on top of it too. I spent a lot of time on miscarriage support forums, and there were a lot of older moms, in their mid 30s who were genuinely scared they might not be able to have a baby, or that being set back a year was going to cost them the ability to give their kid a sibling. I don’t say this to scare you, just to let you know to really weigh these risks carefully so you don’t end up with regrets and unnecessary pain.
Edited to add: personal reproductive health matters alot here. Some women are fertile goddesses and can get pregnant quickly, over and over into their mid 40s. Others find they are running out of decent eggs at 30. If you want to be proactive about TTC without actually trying for the kid, take stock of your own fertility history to try to get a better sense of what might be a smarter move for you. Take note of your cycles (are they regular, normal, like clockwork?) and start talking to your OBGYN/midwife about your age related concerns.