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!
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.