Do you play the Piano? I'm self-taught, learned while working nights at a hotel on the lobby piano.. 
I wish I could play the piano, it is so beautiful! Has it been around for long?
They were invented in Italy in1700 so in the big picture no. No better time to learn then now. 
That seems like a long time haha, but I guess not in the span of all things. Is it hard to learn
The earth is 4.5 billion years old so it's not that old.  The word "piano" is a shortened form of "pianoforte" so it still hold's some of it's original flavor. 
Who came up with the piano
Only the third coolest man ever Bartolomeo Cristofori 
Haha that is an awesome name, are there any instruments similar to the piano
Not really, I mean fortepianos  yes, but they have a smaller dynamic range.