When will people learn that a great footballer is never finished until HE decides he's finished?
Football isn't all about the physical. It's an art form, to some players. Many, many players down through the years have been slow, not very athletic and not very fit, compared to the modern standard, but yet they were able outplay opponents, time and time again.
Cantona, Zidane, Sheringham, Le Tissier and many more great players have used technique, skill, vision and a good footballing brain to devastating effect, without ever being considered, athletic or do much running about the pitch. Giggs falls firmly into that category, these days. He'll always have the technique, skill and footballing brain to bring something to the team, which is why he's lasted so long. If Giggs thinks he can go on for another few years, then who are we to doubt him.
He won't be brilliant every time he appears for us, but none of our players ever are, they all have runs of bad form. It's nothing to do with age.