This is one of those "How long is a piece of string?" questions.
My eldest (6.5yo) was "odd" from a very young age, had her first hyperfocus/special interest from 8 months old to over 2 years old and also had loads of other small indicators and was just not quite the same as the kids I had cared for who were NT but I couldn't put my finger on it (and healthcare professionals put it down to my depression/anxiety seeing things that were not there). Ironically I was a childcare professional specialising in working with autistic families pre-kids and did not pick up on it, I knew she was different but it took my colleagues suggesting looking into adhd when she was 4 and the first paediatric consultant we spoke to suggested it due to a few key issues we had with her, namely being extremely 'over' sensitive, very extreme reactions to relatively simple Halloween decorations at her nursery, hated people having any kind of makeup or face paint or face covering, even just a red button nose painted on people being 'elves' who we walked past at Christmas, or a small masquerade mask, and we couldn't enter shops after October due to santas face being front and center until last year. We think the obvious 'onset' was delayed for her as she was with a childminder (smaller group than the second nursery which she got pulled out of within a couple of months) and then didnt go to school as she's home educated, she started showing the more obvious signs about a year ago, maybe slightly longer (so would have been nearing year 1 if in school). Most auties I've known who have been in nursery/school tend to start showing those signs in reception year, but not always. Every childcare professional she's been with has raised the "have you looked into whether she may be autistic...." question as well, she's had a few thanks to moving and covid. They all picked up on some differences within a few sessions.
Like singleandprouds DD she also cannot imagine her own things; she will mix storyline from things she's watched/read/heard but cannot come up with anything 'original'.
My 4yo, otoh, seemed NT until she was 3ish. We suspect she is autistic with a PDA profile and her meltdowns involve violence directed at her direct family members. We had hoped it was a late terrible twos thing but we are now seeking outside support as it's beyond normal toddler/preschool behaviour and has markers similar to what some of my ex-charges had when they were struggling with emotional regulation. She has other things that are going on too, she repeats phrases she enjoys a lot, some have become regular daily fixtures in our house now! She also chose not to attend a childcare setting when we relocated last summer (I didn't need childcare at that point and the only one she liked were a tiny new setting whose ofsted registration open date were delayed by 3 months so she didn't want to go when it finally opened 😭) and struggles in groups in different ways to her sister, she tends to not ed short bursts in busy places and then to be removed as otherwise later on she suffers more emotional/social come down, whereas 6yo needs a lot of support socially in the moment and needs to be told things directly, which a lot of kids won't do for fear of being 'rude', so I have to intervene a lot to help her navigate social situations, she also becomes obviously overwhelmed and needs social in small chunks. I suspect had she been in school she wouldn't have lasted for long, whereas I think my 4yo would cope in school by masking and we'd deal with the fall out at home. Eldest would need a one to one in school to keep herself and others safe from her impulsive behaviours, 4yo would need to be kept an eye on and taken off for a break regularly (though obviously no resources for that in most schools! And masking so they wouldn't necessarily notice and she wouldn't realise she needs it).
I think it's important to add though that I'm diagnosed adhd and likely autistic, my husband (their dad) is autistic and our wider families are mostly undiagnosed but very likely a mix of neurospicy, particularly our mothers but siblings too.
Noise sensitivity can be part of the wider picture of being autistic or adhd or neurodiverse, but it can be an indicator of other things as well, or 'just' a thing in itself.