It's very open to interpretation. I've heard people say they're as young as fifteen or as old as in their mid-to-late-twenties. Personally I find both extremes, well... extreme, but it's all up to personal preference, in the end.

Oh ho ho! I'd say Rarity was one of my favorites, but I'll be honest here... they're all my favorite. And any and every one that I can convince someone to bring in is a little personal victory for me. So if you go that route (heh) you can bet on me being eager to welcome her in.