Houseplants are important psychologically. They demand nothing from us other than water and light, yet they are a living thing whose existence depends on us. By requiring our care they allow us to shift our focus from ourselves to something else, but at a much lower concentration than pets or humans. They're useful for staving off depression. The hard part is when the depression wins.
Mine started about ten. I'd had a couple houseplants and an aquarium in my room in 5th grade; when we moved in sixth I took the opportunity to plan a large skylight (which I never got - my parents put them in two other rooms but) and hang up a couple 4' grow lamps. By the time I was sixteen I had maybe 25 pots of various foliage, two aquariums and a hand-me-down cage full of finches.
By the time I was seventeen my sister was stealing my shit to sell to her friends, my parents weren't interacting with me unless it was to give me shit, I was a full-blown exercise bulimic and I was trapped.
And I let it all die.
At one point I went away for four or five days and my parents broke into my room because they suspected there were things to be fed in there. They apologized when they realized everything was long since dead.
But they never wondered what the fuck happened, and they never did anything about it.
I like green, growing things. My own little Silent Running in the middle of the fucking New Mexico desert was my escape capsule. And the fact that I couldn't keep it alive still messes me up.
The fact that my parents never gave a fuck made me angry for a very, very long time.