@Mummyof2girlys Please don't worry about your DC's age, I have also heard it's harder when they're younger but my DS was 2 and a half when I applied for him and we just got his backdated award payment.
He's not diagnosed yet either so all I could send in was a initial doctor assessment report with the form- it seems to come down to the severity of which their condition(s) affect them day and night.
Keeping my fingers crossed for you 🤞