It was Tatyana's nanya, who said that she was forced to marry at 13 and her husband was even younger. Nanya was a serf.
Vladimir, Tatiana, and Olga grew up playing together, and Lenskiy constantly refers to that happy time in his life. All three have to be about the same age. There is little difference in age between Tatyana and Olga, so all three are between 17 and 19. There is no mention of a specific age for any of them. Still, this is 19th Century Russia and not 16th Century Verona - they are of proper marriageable age. If Lensky were any younger, his friendship with Onegin would be more of a teacher and protege, which is not likely to result in a duel.
Onegin was about 23 at the start of the poem and 26 at the end of it:
Onegin (once more I rehearse
His story) having killed his friend,
Without an aim on which to fix,
Reaching the age of twenty-six,
Prince Gremin and Tatyana have been married for about two years, as he said to Onegin at the ball:
"Wait, I'll introduce you.-
"Who is she? - My wife.-
"So you're married! I didn't know the wound!
How long had it been?" - About two years.-"
From the text we know that Tatyana was taken to Moscow to find a husband, about one year after the duel, and that's where she met Prince Gremin.