Are strands of gray sneaking into the audience of your hair? And is that crowd thinning? Did you get a walking stick for your last birthday? Might it have been your last birthday, for all the aches and pains you suffer? You might be on your way over the hill, a rough ride that's more like the Scrambler than a roller coaster. For a taste of the fountain of youth, go see the symphony and you'll learn that everything's relative. And if you spot a relative there, use him as an example of how you might look at that age. Powell Symphony Hall puts aging in perspective. Heck, a good percentage of the audience needs help getting up for a standing ovation. (Hey, we're only joking; pardon our juvenility.) What will happen when this audience moves on? Are there new spectators waiting in the wings? Who knows? -- but if you have to go to the symphony to feel young, avoid loud concerts. Unless you want to feel like the father of rock & roll.