How old is RGB?
Due to us not knowing much about this particular monster, we can only assume of how old he is...this is an estimate and not a very good one... But I believe he is over 100 years old, the reason being with his quite posh way of communicating and the era of his clothing, now I know that he is wearing a facade and it's probably a disguise but hay it's the best I've got
If we were to take a took at his language and way of speaking, we can clearly state that he does speak quite formality, which could indicate which era he would have came from, most likely from the late 19 to early 20th century, he has also mentioned quite a few interesting sayings within the comic which, I quote "spit spot" , this might not be a major clue, but this saying comes straight from the movie Mary Poppins, that movie being set in 1910.
His clothing also give a hinge indicator of which era his from, which were to be the Edwardian era (1901-1910), you can see this by the picture below
I mean look at it, match his style of clothing doesn't it? (Apart from rgb's looking like it was splattered with paint but whatever)
Now I know I don't have further more explanation but eh I've tried, also I can't think of any other theories to come up with so I might turn this into a Tpoh rant book or something.