This is one of those decisions (whatever you decide) that you won't know was the right/wrong decision until years down the road.
Two of my 3 kids have ADHD and those two were both born in the spring - so not really candidates for delaying K. My older DS was diagnosed in K (before his 6th birthday) with ADHD/MERLD and got his 1st IEP in K. He'd been diagnosed with anxiety the spring before K. He did not do well academically in K but was positive about school, had friends and no behavioral issues. He was also notably less mature than his peers and his language/communication and emotional regulation were notably delayed. Younger DS has a similar profile to older DS but was delayed enough that he was eligible for the non-cat special ed preschool. He, however, learned to read in K. I had wanted to hold younger DS back a year but was told his IEP would ensure appropriate support and if I held him back, he would still not be eligible to continue with the special ed preschool (which is offered by the school system).
The kids are now in HS/MS. In hindsight, it was the right decision not to hold older DS back - but that wasn't clear until he was in MS. By that time, with the exception of math, he no longer needed special ed services/support. He seemed more immature than many of his peers at the lower/middle levels of ES but this was less noticeable as he got older and, per HS teacher report, he is one of the more mature kids in his grade. Younger DS, on the other hand, I wish we'd kept him back. He is maturing and developing but still noticeably more immature than his peers. I think he is one of the few kids that really would have benefited from additional time. For most kids, like older DS, they tend to even out by the time they hit MS.
Really helpful, wasn't I?