Pirlo started at 16 and is still going 20 odd years later. Rooney like Pirlo has a top football brain and can be used in other positions as he gets older. These types of players don't come around often.Indeed, rooneys basically in his mid 30s in terms of conditioning.
His physique and body type combined with him playing competitive football regularly from the age of 16 means he was always going to age quicker than other players in his age group.
I'd love to be proved wrong and see him regain form at the club he loves, would be a great story... But when your body's done, it's done. No amount of will power can reverse that.