Downplayed: Bob is a senior colleague of new recruit Alice, a few years older and more experienced, and is training her to do his job when he retires. (Yeah, that'll happen)
Justified: Bob is an Old Master in a Dangerous Forbidden Technique, and in order to keep it alive but obscure, it is taught to one carefully selected apprentice over the course of their entire life. When their master dies, it will be the student's job to find and train a new apprentice.
Inverted: Bob and Alice are hackers. Alice is a child prodigy, while Bob is her tag-along legal guardian who acts as The Watson as she tries to teach him her skills.
Subverted: Everyone assumes Bob is Alice's mentor, but they're actually pretty well-matched in skill and knowledge - he's just older. They each learned their jobs from different people.
Double Subverted: ...However, it gradually becomes clear that Bob's wider experience of the world means he still has a lot to teach the arrogant, hot-headed, single minded young prodigy he's partnered with.
Bob finds and begins to train an apprentice, only to have them die in front of him, Once an Episode. Apparently he's not a very good teacher...
Every single profession requires new recruits to shadow an experienced mentor from first hiring to qualifying as a Master themselves - from deep-cover secret agents to jet pilots, from prostitutes to Burger Fools.
Zig Zagged: Bob is older than Alice, and they're very much an Odd Couple, but they each have their strengths and weaknesses and which one is "teaching" the other varies from situation to situation.
Averted: Bob and Alice are roughly the same age and the same level of competence.
Lampshaded: "Why do you guys always come in pairs? What are you, the Sith?"
Invoked: Bob doesn't want his knowledge to be lost when he dies, so he's training a replacement on-the-job; also, he's too old for this kicking-down-doors and roughing-up-bad-guys shit, so he likes to keep some young blood around to fetch and carry.
Exploited: Bob's old rival knows his tendency to team up with Hot-Blooded, less skilled partners, and plots to lure the apprentice into a trap so that The Dragon can take on the elderly master alone.
Discussed: "Come on, there's no way you came alone, old man. Where's the other one? I'm not killing you just to be ambushed later by some vengeance-mad protegé..."
Conversed: "Why don't these guys have more than one student at a time? All it'd take is one dropout or untimely death and this all-important ancient secret would be lost forever..."
Deconstructed: Bob's ancient order demands that a master only train one student to take up their mantle, but this rule combined with the difficulty of finding suitable apprentices has meant that the Order has dwindled to only a handful. Bob has spent his life trying to find someone worthy, and every time a student has failed to live up to his expectations they've fallen victim to He Knows Too Much. A lifetime of purposeless searching broken periodically by being forced to kill those he has grown close to has made Bob lose faith in his work, and when his latest hopeful Alice fails the Secret Test of Character he resolves to train her anyway, and earns the enmity of the remainder of the Order, forcing them to go on the run together...