Where Are The Crayons?

In the show he's shown with a cutiemark when Twily was a filly but reading and all (7-9), so this is much earlier, when Twily was a baby but not newborn. I'd say 5 years earlier from the show's filly Twily, here. If SA is in his middle 20s now, he was in early 20s or late teens, here he has to be in his teens (13-17, not more by all means).

MLP Wikia doesn't say anything about when his CM, and there are nothing in the show where he doesn't have it. My money is on a meeting with Celly in his teens or preteens for a possible guard or even guard captain where he got it, or automatically gaining it even earlier, quickly prompting the Sparkles to talk to Celly. Or worse yet, he had to protect someone or something and that's when, but nothing implies of any wars or something where SA would had been while Twily was baby or filly; Equestria is too peaceful that it was some non-attack or non-emergency when it happened.