Hi. As others have said, usually at 18 months they would leave them anyway as all children develop at different rates and even if you suspect autism, they won’t do anything until age 2-3.
We have had concerns about our DS3 from around 18 months - 2 years of age. He started falling behind with milestones. Mainly we noticed this with his speech and general communication. He is now 3 years and 10 months and I would say his speech for the amount of words he can say is about a 2 year old (literally just 2) but his ability to use and put words together is less than this. He can sometimes speak in 3-4 word sentences, but More often it’s 1-2. I though he just had a speech delay initially. At 30 months, the HV said she had no concerns. Doesn’t answer to his name, doesn’t make eye contact with anyone but me, won’t point or look at things pointed out to him.
He started pre-school shortly after his 3rd birthday and after 3 months I asked them what they thought about his speech. He had progressed and did talk much more there than he does at home but they had concerns. Next thing the HV is visiting me and within 3 minutes of sitting down she told me they were querying autism. Could have knocked me down with a feather! At first I was completely adamant they’d got it wrong. But as time has gone on, and I have educated myself on the signs of autism as a spectrum, I think he is. He is on the assessment pathway now and started with SALTS on a 1-2-1 basis (did group sessions initially but didn’t work for him as too much noise and distractions), had initial report from paediatrician, had hearing and eye tests (both normal), podiatry for balance as he trips a lot and sways when running, (has heel cups) and portage for support at pre-school. It’s not a bad thing to make yourself aware of what the signs can be and what you can do as you are then well prepared to help your DC.
Just be prepared for them to say, they will see how he is in 6 months or so.
My nephew didn’t walk until he was 23 months and had to be massively encouraged even then. The paediatrician said he had really flexible ligaments. He also used to wrings his hands when excited and still does to this day (15!) but he isn’t autistic.
Bottom line is, all kids are different and you’re not wrong to be concerned and be proactive in this. Hope you make some progress with pushing forward with this.