You might say "successful aging" means finding another candle on the old birthday cake, but I don't believe success can be defined in such simple terms where quantity equals quality. I think successful aging means something more complex, like coming to grips with your mortality and the small, but important role you play in life. You also have to wise up about your dreams. I abandoned my childhood dream of becoming an astronaut. I also gave up the extremely unrealistic dream of owning a vintage Italian car that didn't leak oil and make me swear. Does that mean I'm successfully aging? Probably not, but at least I understand my role in the grand scheme of things. My role is "being a small but important cog in a big, complex machine" - a machine that is probably made in Italy and leaks oil. I may be a cog, but I'm a not a stupid cog.
Successful aging also means remaining sane as each year passes. I know it's hard to remain sane when you would rather run screaming through the neighborhood shouting, "If one more hair falls out of my head, I am going to run screaming through the neighborhood shouting about it!" Of course, if you do run through the neighborhood shouting that warning, you probably don't need to do it again when the hair does indeed fall out. That would be the epitome of unsuccessful aging and somewhat redundant. Successful aging means staying sane despite the odds. I'm aware that this is a debatable point, and that the insane might be the happiest. But let's be good little cogs and ignore that for now.
Finally, successful aging means accepting your mortality. Then you can do things you would never have done before. You can buy flannel sleepwear; you can join AARP; you can snore and drool all night long. And there's more. You can complain about body parts that hurt and body parts that you don't own anymore; you can also mumble. Mumbling is the best! I know what you're thinking, but just give in. Eat your birthday cake, put the candles in your flannel nightshirt's pocket and mumble about your aching shoulder or calf muscle. When you age successfully you earn that right!