It's takes a lot of collaboration with our local early intervention (what we call our birth-to-three programs in Mass) providers to make those referrals early. Part of this was our decision to pretty much limit our assessment practice to under 30 months. Just increasing that age limit by 12 months means hundreds of new referrals and and and adding a year to our wait-lists, which would mean kids are at least 30-42 months by the time I'd see them. It was a difficult decision (lowering max age limit) that probably resulted in a cohort of kids not being seen (we are one of the only games in town), but now we get the young ones referred as soon as any potential ASD signs are noted. No more "waiting to see if they grow out of it" by the early intervention staff (which they rarely do), which in turn has somewhat influenced pediatrician referrals. It sucks that there aren't people out there doing evals on the preschool and early elementary kids (unless you can afford private pay, which young families can't), and turning away desperate families sucks even more.
It does mean I've seen a really young (12-14 months) kids where I just couldn't make a diagnosis or- if motor development too delayed- use an ADOS. For those kids I either do the intake and make a plan to see them when they are a little older and natural development has has more of a chance, or do the testing and get some baselines to compare them against when they come back. I'm about 50/50 with the really young ones who I don't give an initial dx- they come back in 3-6 months and have then get an ASD dx and half don't. End of the day, we get young kids in fast (1-3 month wait) and get them into treatment soon after.
From a practical standpoint, it means that I almost exclusively work sitting on the ground, as the littles don't/can't sit at my little table, and my knees are definitely feeling it. It also means that I can get out of practice with the older kids (36-48 months is old in my books), so I will occasionally scheduled an older kiddo for a reval (or sometimes see an older sibling of someone I diagnosed) so that I keep up my skills with that age group and some of the "big kid tests" (WPPSI, CASL, NEPPSY, etc.). Also, parents of very young children also tend to be very young, thus more likely to be low SES and having Medicaid for insurance. That means that if my company wants to pay me fairly (which, imho, they do), a stand alone, toddler ASD diagnostic clinic is not viable, which is why you see them (us included) increasingly as part of a bigger ABA service provider. It also means that I am hyper vigilant about doing my job, billing what I'm expected (and more!), and doing other "value added" stuff, such as clinical supervision, staff training, senior clinical consultation, and teaching in affiliated grad programs.