Approaching the half-century milestone? Know someone who is?  Richard Smith€s wildly popular books have made America laugh about everything from wedding-night jitters to weight loss. Now he turns his comic talent to a subject he€s experienced himself: climbing the half-century hill.  Over the next fifteen years, 52 million Gen Xers will celebrate their fiftieth birthdays. Whether you€re about to hit the big Five-O or already have your AARP card, there are certain signs that you can€t ignore. Because even if you feel like you€re at the top of your game, you know you€re 50 when . . .  € Your main form of aerobic exercise is getting up to find the remote € You think a mosh pit is something found at the center of an exotic fruit € €œPerformance anxiety€ refers to golf . . . and bingo night € You read obituaries not to see who died but how long they lived € Twice in one night refers to bathroom visits  So go ahead and spend your kid€s inheritance on a Porsche (and liposuction so you can fit into it)€"you€ve got another 50 years to go!