My grandsons are 5 & 2 and both have been in my hammocks since they
were old enough to hang on to one. My rule is that if you are big
enough to get into one, then you are big enough to sleep in one.
I call them (the grandsons) Tarzan and Cheeta, they do play on it but
then, they hang and swing from trees too. We encourage active play in
all the grandchildren. They are allowed to swing on the hammock but
not jump in it. That's the only rule.
They love the hammocks as much as we do and I expect them to be having
their own all throughout their lives.