No, all Cursed Children are not 10 years old, it is most likely that Cursed Children are still being born.
In the light novel the girl who opens the manhole cover for Rentaro in the Outer District is stated to look around the age of 7
She was probably around seven years old, and she peered at Rentaro with a puzzled look on her face. Her eyes glowed red.
While not conclusive, the following passage also seem to imply that Cursed Children are still being born, which is probably part of the reason the new Gastrea Law is being proposed
Just a short while ago, it was normal for Cursed Children to be delivered in secret alongside a river, then killed before they could even open their eyes
While "a short while ago" is hard to pin down I doubt it only refers to 10 years ago. This might also be the reason why we see so few younger cursed children, most are drowned shortly after birth. 10 years ago parents didn't know that their children would be born as Cursed Children but in more recent years it is probably expected that a large amount of children would be born that way which probably explain why most parents kill their children before they can even open their eyes.
The second light novel again confirms the fact that Cursed Children are not all 10 years old but are in fact also younger
Born at almost the same time as the Great Gastrea War ten years ago, the Cursed Children were ten years old or younger.
Now, the Cursed Children wiki page states that Matsuzaki states that all Cursed Children are 10-years old however I cannot find this statement in either the light novel or the anime so I would take it with a pinch of salt.