Harvard graduate Alexander Kerfoot will make his NHL preseason debut Tuesday playing left wing on a line with veterans Nathan MacKinnon and Mikko Rantanen. The thing is, Kerfoot is the old man of the Avalanche’s top line. He’s 23. MacKinnon, who is beginning his fifth NHL season, turned 22 on Sept. 1, and Rantanen — who led Colorado with 20 goals last season as a rookie — will be 21 next month. Still, it was Kerfoot who felt like the youngster after Tuesday’s morning skate at the Pepsi Center, where Colorado will host the expansion Vegas Golden Knights at 7 p.m. “It’s a pretty cool opportunity. They’re pretty special hockey players,” said Kerfoot, a 2013 New Jersey Devils‘ draft pick who became a free agent Aug. 16. “Obviously, I’m really excited but I don’t want to get too nervous; playing with those guys can be nerve-wracking at times. I just want to play my game and stay focused — try to keep things simple but also make plays and give them the puck.” Kerfoot, who served as Harvard’s captain last season and tied for a team-high 45 points in 36 games, is being given every opportunity to make the Avalanche out of training camp. He has only practiced with Colorado’s top forwards — including Matt Duchene, Gabe Landeskog and Tyson Jost.