From my old French lit professor:
"Ton français est devenu d'une aisance admirable" ("Your French has become admirably fluent"
From a francophone professor who is very difficult to impress, a big compliment.
From a co-worker who works "under" me on my project:
"Really? You're 26? I would have thought older. Not because of your physical appearance, but because of your competences. [...] You should be proud of yourself."