The r-slur is targeted at intellectually disabled people. It’s not something for you to reclaim. When people call non-intellectually disabled autistic people that, the insult is that they’re comparing them to intellectually disabed people.

We know that human brains are still developing until around 26 years old

this isn’t true by the way.

The human brain, particularly the prefrontal cortex, does not reach “full maturity” or "full development at any particular age (e.g. 16, 18, 21, 25, 30). Changes in structure and myelination of gray matter are recorded to continue with relative consistency all throughout life including until death. Different mental abilities peak earlier or later in life.

Text taken from wikipedia’s list of common misconceptions. Here’s a study on how the brain changes throughout life!

I’ve had people misgender me and try to convince me not to transition and then go claim to be trans allies because they don’t call people slurs and will say that killing trans people is bad if asked

It’s not about “maturity” or “protecting kids”, they don’t want anyone to transition. They just focus on youth because it’s easier to take human rights away from people who have less rights already.