This is the 14686th most frequent Russian word.
перестают
"Перестают" means "they stop" or "they cease."
In this sentence, 'перестают' (stop) is used to express the cessation of an action, specifically the act of believing in something.
Люди перестают верить в чудеса.
People stop believing in miracles.
Here, 'перестают' (stop) indicates the discontinuation of the activity 'играть в игрушки' (playing with toys) as children mature.
Когда дети становятся взрослыми, они часто перестают играть в игрушки.
When children grow up, they often stop playing with toys.
In this case, 'перестают' (stop) refers to the change or cessation of the state of the trees being green during fall.