dd1 was dx/d at 2.8, which seems quite early on, and lots of people assume it must have been a straightforward dx, but it was anything but!
(well, it was pretty clear and straightforward to me)
I first raised concerns with my HV when dd1 was 8 months old. yes, really. she was pretty extreme, though, and very shut away at that age.
I was still left to "wait and see" though, and nothing was done, not even a referral on until dd1 was 16 months old. we were then in the system, and it took a further 4 motnhs to see anyone, and so on.
finally, after seeing at least 4 different community paeds, and getting referred on to the social & communication clinic (with assorted SALT/portage/genetic screening along the way). within 20 mins of our social and comms appt, the paed and SALT looke dat each other and said "I think there's little doubt" - well, I wish all the others had been so sure!
so it took us 2 years, really, despite dd1 being severely ASD.