“Quarter-Life Crisis”
A quarter-life crisis is uncertainty and anxiety about your direction in life, often around your mid-twenties. The name imagines a hundred-year lifespan and a crisis one quarter of the way through it. It plays on mid-life crisis and describes Tina's worries.
“Nobody our age has it figured out.”
Have it figured out informally means to have found the answers or know what you are doing. It loosely refers to life or the future. With nobody, Frank reassures Tina that no one their age understands everything.
“Nothing you choose now is set in stone.”
Be set in stone means to be fixed and unchangeable. Nothing reverses that meaning: today's decisions can be changed later. Frank reassures Tina that one choice does not determine her entire life. Be carved in stone has the same meaning.
“Nah, they're faking it. You're only seeing the highlight reel.”
Fake it means to pretend to know what you are doing. A highlight reel collects only the best moments, originally in sports footage. Here it compares the successful-looking parts of friends' lives to selected clips, a common image in the social-media era. Nah is an informal No.
“Deal.”
Deal. comes from agreeing to a transaction and informally accepts a proposal. Tina warmly accepts Frank's suggestion that they figure things out together.