Envy does not remember how old she is. Her memories, just like Lust's, are vague as if they are not her own memories, but a story told to her.
Still, she's sure that never anything like that happened in her entire life…
Since Envy remembers, her life has always been about helping her people, getting powerful hosts, and generating demonic energy with them to get even stronger.
But all that, this mysterious power that she has, is about her current host, that is, she can't do anything on her own. She needs her hosts to be willing to do certain things for her to benefit from their demonic energy.
To depend on someone else like that is certainly annoying, but over time Envy has learned to manipulate people to get what she wants. That is her nature, and she accepted it.
Then, that day came, precisely the moment when Envy, for the first time, generated demonic energy in a way without the help of her host.