I think there are levels of maturity in all things... including love. But like any emotion, there's not much account for maturity or sense in what makes people feel the way they do.
In this world, there are people who fall in love with animals, inanimate objects, celebrities... all sorts of people who CAN'T return their feelings in any sort of sensical fashion. But they still end up feeling "love" one way or another.
It's totally possible to fall in love over the internet. It's not SMART, mature or necessarily based entirely on truth (the internet makes it too easy to tell white lies, send touched up photos and edit your statements)... but plenty of people have done it and plenty more will.
In general, think the internet should be used sort of like a telephone: as a suppliment to real life. You meet folks and like them... then you go out of your way to meet them and make those relationships "real". That's not necessarily immature: it's actually pretty freaking brave (since it generally requires travel, potential dissappointment and being totally out of your element).
My parents both remmaried people they met on the internet. Half of the relationships I've ever had began as penpals, tutors I met on the net, or just online buddies. As I've gotten older I've gotten a lot less likely to "fall" for someone texting me... but when I was younger, I did the long-distance thing once or twice myself.