My heart leaps up when I behold A rainbow in the sky: (1-2)
From the jump we know that the relationship between man and nature in this poem is a friendly one. Our speaker doesn't tell us about the rain that likely precedes these rainbows, but instead he idealizes nature, choosing to focus on its beauty rather than its wrath. Wordsworth lived in England, where it rains a lot—but all we hear about in this poem is how wonderful it is to be able to look at rainbows.
So was it when my life began; So is it now I am a man (3-4)
We doubt the speaker could remember how he felt about rainbows from the time his life literally began, but the idea is that they made his heart leap up when he was young, and they still do. We bet the speaker is guessing that some people will scoff at the first two lines, finding them childish. So here he's saying: yeah, sure, I got excited about rainbows when I was a kid—but I still get excited now that I'm all grown up too!
So be it when I shall grow old, Or let me die! (5-6)
Not only does the speaker want his audience to know that rainbows are more than just a childish joy for him, he goes so far as to say that if he stopped loving rainbows, he'd just as soon die. Why bother living, he seems to ask, if you can no longer appreciate something so beautiful? He loves nature—to the extreme!