Now, of my threescore years and ten, Twenty will not come again,
Okay, after a stanza describing the tree itself, the speaker shifts to a more reflective mood here.
As well, we get some… math? In a poem? Argh. All these numbers here are a little confusing, so let's take it piece by piece.
The speaker says that he has threescore years and ten, and twenty of them will never come again. Pencils ready? Graphing calculators on? Green visors in place? Let's do this:
So, a "score" is just an old-fashioned way of saying 20. (You may be familiar with that idea from this famous speech by Abraham Lincoln.)
This means that "threescore" is just three times 20, or… 60. (We had to take our socks off and get out the toes to calculate that one.)
Now, if you add "ten" to "threescore" (60), you get… 70. Ta-da. So, the speaker is seventy then?
Um, not exactly—and that's where the confusion comes in.
He's imagining that after, everything is said and done, he will reach the ripe old age of 70. You could paraphrase it like this: "I will probably end up dying when I'm about 70."
So, he guesses that he will live to be seventy. He also knows that his current age ("twenty"—20 years old) will never come again.
We know, we know. This is a very roundabout or confusing way to say "I'll never be twenty again," but it's late Victorian poetry folks. And Victorians couldn't say very much of anything without using roundabout phrasing.
And take from seventy springs a score, It only leaves me fifty more.
Fun—more math awaits us.
This time the speaker uses the same roundabout method to calculate how many years he has left of life:
Assuming that the speaker will live to be 70, if he takes away the 20 years he's already lived, that means he's only got… 50 years left ("it only leaves me fifty more").
As in the two previous lines, the speaker again uses that word "score." Spring comes once a year, so "seventy springs" is like saying seventy years.
Now, there'd be two ways to react to this realization. Way #1: "Woo-hoo! I've got 50 more of springs to enjoy! I'm totally set." Way #2: "50?! That's it? I better hustle up and get living."