How do you tell if your child has a younger mental age

My son was diagnosed with autism at around 18 however its hard to.know whats autism.and what's possibly caused by his mental age being younger than what he is...for example with everything he will say oh yes I'll just do that..for example I wantdnto renew his passport as if he buys a ber online in the shopping they need photographic evidence at door etc...anyway as we r isolated and we don't have any family and it was covid we couldn't even ask.neighbour to sign the photos for the passport as they had covid...so my son said we'll take it to a solicitor hell sign it...I try to suggest this might not be possible.but thought I'd go along with it as I wanted to ty to trust him and give him independence etc..anyway it was a disaster when he was in the solicitor office the solicitor had to explain.over and over how he couldn't sign the photos but he just didn't  understand..anyway he has this attitude with everything he doesn't seem to grasp the details of things which he really should as he is 24.now..I came on here to ask ho I can tell if he is developmental delayed as.i don't want to out him.througg anymore crap with doctors as finding out he was autistic made him suicidal and I've only just got him stable again and I think iff he did find out anythingnelse ia different about him he will get depressed again.