It hugely depends on the child.
I have 2 boys, one nearly 13 one 6. The 6 yr old’s handwriting is genuinely better than the 13yr olds. 13 yr old has mild ADHD and has always had poor fine motor skills (mixes capitals, writes certain characters back to front) whereas the 6 yr old has always loved mark making and is advanced for his age.
Thet said the 13 yr old is obviously much better at descriptions, different writing styles etc which is what starts to count as they get older.
If you’re worried, speak to school and if they don’t take you seriously speak to them again. But generally these things come out in the wash as the kids get older.