A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Once it becomes a creature, it will not change back into an enchantment if your opponent's life total goes back above 10.

Once it becomes a creature, it is no longer an enchantment.