The retired adventurer (or police officer) makes a great significant NPC in a campaign. They can have a worldly wisdom that comes of years of experience, and a surprisingly well-sustained reserve of physical prowess, hidden by a middle-aged spread and greying hair. They also come with a good excuse for not accompanying the heroes or getting too involved in their adventures ("I'm retired! I'm not going galavanting all over the countryside!"), or a good excuse to accompany them and get involved if necessary ("One more outing - ah, how I've missed this!"). They can be pretty much whatever the situation requires, which is always a good thing to have up your GMing sleeve.
Lando: Now look here, Lord Vader.
Darth Vader: Yes?
Lando: I know you think this guy's Han Solo, but I can vouch for him.
Darth Vader: And why should I listen to you?
Lando: I was a duly appointed officer of the law before I retired.
Darth Vader: Do you also vouch for Senator Princess Organa?
Darth Vader: Solo's companion is the fugitive Senator, and Rebel leader.
Lando: You're kidding! Aide, look up the bounty on her.
Robot: Robot, sir.
Lando: But still. Forced surgery on innocent civilians. This never would have happened under Chancellor Valorum.
Darth Vader: You'd be surprised.