it can improve with age, yes.
however, at 18, my DS has eaten the same exact meal for breakfast, and lunch, for 14 of those years and a variation of the same 4 or 5 for dinner. he will not add to it. He also ONLY drinks water since he was 6.. thanks to those 'healthy eating' assemblies that demonised squash/juice.
ARFID may only have recently been 'officially' diagnosed, but its been well known as a problem since my son was about 7, although back then it was lumped in with sensory processing disorder.
We were offered therapy, but the dr was on sabbatical and no-one bothered to ever pick it back up with him as they never returned to the UK....
The main take away for anyone not familiar with it who might be reading the thread is that getting fluid into ARFID kids is the main importance, the type of fluid, be it squash, water, juice, tea/coffee/hot chocolate isn't the important thing.
Rule 1 for Arfid is the old adage 'fed is best' and nutritional concerns are the last of your worries. Let them eat what they'll eat and support with multivitamins.