It really depends.
Mentally, you could be ready at any point from 16 onwards. Alternatively, you could feel desperately unready at 40.
Physically, it's a bit more constrained. I think medics generally agree that you're most fertile before the age of 30 and that it starts to decline from there.
The difficult part, to my mind, is trying to get mental readiness and physical ability to both occur at the same time. I've got friends who tried for 10 years to conceive (started at age 20) or who got pregnant easily at the age of 40.
I do think it is astonishing that we, as a society of women, mostly just wing it. I'd love it if all women had the option to check their fertility level at the age of 18 and could find out if they were super-fertile (in which case, delays might be fine) or not super-fertile, in which case they could make an informed choice on the whole 'wait-or-not' debate. My step-MIL has no biological kids and she thinks it's cause she put it off too long :(
So in summary, maybe 35 is too old for some but it certainly isn't for all.
Sorry, essay!!