I'm in my 50's and I have been quietly approaching famous people for years to tell them I appreciate their work. I have never received anything other than a gracious reply in return. Over the years, I have approached actors, writers, politicians, composers, and everybody in between. IMHO, the key is to be quiet, polite, AND BRIEF! Something along the line of 'I'm terribly sorry to disturb you, but I just wanted to tell you how much I enjoyed your performance/book/score for (fill in the blank)'. AND THEN STOP. Unless you have interrupted them in mid-sentence, only the most arrogant snob would refuse to at least say 'Thank You'. If they are interested in talking, let THEM continue the conversation. Otherwise, offer a polite 'Good Evening', 'Enjoy the Show', or whatever is appropriate, and go back to what you were doing (presuming what you were doing was not standing on a chair shrieking 'Look everyone, it's Stephen Sondheim!')
Now, having said all that, there are three occasions where I would not approach a famous person 1) they are sitting and I am standing (theatre, restaurant, airplane, etc), 2) the men's room (for God's sake, let them pee in peace), and 3) I have nothing nice to say to them (needless to say, I could share an elevator with George W Bush and I wouldn't even acknowledge his presence).
Otherwise, I say go for it.
'Our whole family shouts. It comes from us livin' so close to the railroad tracks'