It’s not unusual for opening scenes to be either flashbacks of a character’s past, or the very end of the story. A bit of a ‘this is where I am now, and I’m going to tell you the story of how it happened’.
I believe YOI actually does both.
We have a younger Viktor skating, who turns into an older Viktor.
The younger, long-haired Viktor is clearly of the past, but it’s not a concrete memory (Yuuri’s there too). It embodies how Yuuri has watched Viktor skate from a young age.
When Young!Viktor skates, he gives Yuuri his back, he doesn’t face him. He doesn’t know him, they’re not on the same level.
But when he moves into the shadow and then we have the older Viktor, he’s looking right at Yuuri. He knows him, acknowledges him.