How old was Frost when he made his momentous decisions?

It is generally agreed that Frost (speaker of the poem) is a young man when he chooses one of the two roads. The final stanza makes this clear: 

I shall be telling this with a sigh 

Somewhere ages and ages hence: 

Two roads diverged in a wood, and I— 

I took the one less traveled by, 

And that has made all the difference. 

He says that he will be retelling the story of his choice...

It is generally agreed that Frost (speaker of the poem) is a young man when he chooses one of the two roads. The final stanza makes this clear: 

I shall be telling this with a sigh 

Somewhere ages and ages hence: 

Two roads diverged in a wood, and I— 

I took the one less traveled by, 

And that has made all the difference. 

He says that he will be retelling the story of his choice years later. So, he is attempting to foresee how he will view his choice as an older man. It is not overtly clear exactly how old he is when he chooses this path. But considering that this idea of choosing a path is metaphoric, it could refer to any significant life choice at any time during his life. Since he is contemplating how his older self will judge his choice, we might suppose that he chooses during his twenties or thirties, a time when many people make significant life choices such as careers, marriage, and children. 

Note that in that final stanza, he says he will be telling the story of his choice "with a sigh." This is the key to the poem. The two roads looked the same: "the passing there / Had worn them really about the same." The "sigh" indicates that he might regret his choice and just pretend that he took the less traveled road. In other words, he hopes he will have taken the road less traveled. He can not know for sure at the time because both roads look "about the same."