To celebrate today on the Beef, Bacon, and Beer server, some questions were asked about people's Shepards and their N7 experience. When did they know they made N7? How did they feel when they got it? Did they have anybody special there when they got pinned? How old were they when they were made N7?
So I figured I'd post my answer for my primary mshep.
Mark is 2 years older than canon, so he's 24 for Elysium and the Blitz, not 22. Some universes he's there but not the Hero, some he's not at all. But always in 2176, his mission in the Hunted that got him his invite to the Villa happens, and that's what starts the whole journey. He takes about a year or so to go through it, and I'm of the old school canon where N6 is the last proper assigned rank and N7 is awarded for going above and beyond in the field. He is 26 when he makes rank.
Universe depending, he's either newly N7 for Torfan and that's his first assignment, or he's awarded N7 for Torfan, and Torfan is always what catches Nihlus' eye for recommending him to the Council for Spectre.
As for when he knows he's N7, it doesn't ever really sink in for him at once; it's in small increments. Small snapshots of clarity about it. Once was when he was told, once was when he was pinned, once was when he got access to new armor and weapons, once was when he was wearing the blood and bone stripe for the first time, once was when he was in command of a battle and it clicked 'I can do this, this is why they chose me, this is what N7 means', and once when he was sitting in meetings for strategy that he thought was above his paygrade until people started asking his opinions and listening to him.
When he is pinned, Anderson is the one who pins him and tells him “Glad to see my recommendation was correct, son. You proved me right.” He knows professionally who Anderson is but had no idea that he'd been flagged and was being watched by him.
I also HC that an old tradition of 'blood pinning' happens, even though on paper no it doesn't how dare you insinuate. What it is is the pin backing isn't put on, so the pin is affixed to the cloth and isn't secured, but then is lightly punched or tapped into the person's chest, making them bleed. Blood oathed to the unit, blood sacrifice to show worthy of the designation, blood offering for the blood to be shed in future service.