The idea of Wordsworth's, "My Heart Leaps Up," is that life isn't worth living if one does not have an intimate relationship with nature.
The speaker's heart metaphorically leaps up when it sees a rainbow in the speaker's present, as it did when he was a child, and as it will in the future when he will be old--if it doesn't when he is old, he might as well die.
And poetry is not that subjective. An intelligent interpretation of a poem must stem from evidence within the poem. There's nothing in this poem about paintings or loved ones or about growing old well. The poem's about having an intimate relationship with nature every day of one's life, whether one is a child or a man.
From ny point of view (as poetry is so subjective) the line in the poem "My Heart Leaps Up" by William Wordsworth is the line "The Child is Father of the Man." The poet is saying that childhood is the most impressive time of a person's life - whatever happens then influences the person for the rest of their life. Luckily for William Wordsworth, he recognises his fortune in being born in such a joyfully scenic landscape and the delight this gives will last right throughout his life - and maybe beyond. It will be expressed over and over again in his poetry to be shared with others - it has formed his character and runs through him. Sadly, some people are not so lucky and their formative childhood influences are negative.
To me, there are a couple of important themes in this poem.
First, there is the theme of the love of nature. The speaker's heart leaps up not when he sees a painting, or someone he loves, but rather when he sees something beautiful from nature -- a rainbow.
Second, there is the idea of hoping to age well. The speaker is hoping that his way of thinking, his emotions, will not change as he grows older. He hopes that he will still feel the same way he felt as a younger person.