I did French to A-level (age 18) and German as part of my degree. I wouldn't say that I can speak either of them, but I can watch German TV and get almost every word, and I can communicate in German if I have to, but always painfully aware of how many mistakes I am making.
I started when I was about 15, but only really made a serious effort when I turned 18 and joined JEB (the young-adult wing of EAB).
I was interested in the idea of a constructed language and how that might work. But mostly because I don't really have the patience to learn all the hundreds of little rules and endings and so on that you need to really master another language. I can remember a time in my first year of uni where I got back an essay I'd written in German that was covered in red ink with all my little mistakes - although the meaning was clear, a good proportion of the endings and articles were wrong. On the very same day, I'd asked @Tim to help me with something I'd written in Esperanto, and his response was something along the lines of "you made one mistake here, and this part could be worded more eloquently, but otherwise it's pretty much perfect".
I have a good collection of Esperanto music, which includes some of my favourite albums - so they get listened to a lot, and I go to a local meetup once a month. I have a few friends that I chat to online in Esperanto sometimes. Otherwise, not a huge amount.
My knowledge is pretty good, but I often stumble over my words when I speak - nothing that wouldn't be fixed by a few weeks' regular practise. I don't have the feeling of being self conscious about making mistakes in every single sentence like I do in German.
It's mostly a niche hobby, and is likely to stay that way. The main incentives that people have to learn a foreign language (economic, romantic, etc) don't really apply to Esperanto, so it's really only people with an interest in languages for their own sake who learn it.
Esperanto is an international language, by multiple definitions. English is also an international language, by some (but not all) of the same definitions and by some other definitions too.
That it is a successful general-purpose constructed language. By successful I don't mean that it achieved whatever lofty goals its early speakers may have had for it, but that it has a sizeable community. Something like Pandunia doesn't really have a community beyond its own official groups. The only other constructed language that I might consider "successful" by that definition is toki pona, but that isn't a "fully-fledged" general-purpose language in the same way.
No. It's just a hobby: something I do, not something I am.