The speaker expresses his wonder and unhappiness at the fact that he is no longer young. He now has to pay attention to his body, because he can no longer go out and race carelessly along cliffs or beaches without a worry. He can no longer be unconcerned about the weather. He states that his body has done him a "grievous wrong" by getting older.
He notes that he sees the changes in himself: his hair has gotten "silvery," his body has changed size, and his walk is now "drooping," or bent over. He calls this state of life a "strange disguise," meaning that despite all the evidence of his body being old, he still feels young.
His sense that his youth has not left him is encapsulated in the following lines:
Life is but thought: so think I will
That Youth and I are house-mates still.
He has, in other words, made a mental decision that life is what we think it is: we can make it what we want it to be. Therefore, he has decided that, because of his belief that he is young, that he and his young self still live together.
The poem makes clear that this is not an outward situation: outwardly, there is no doubt but that the speaker is an old man. However, because he feels young inwardly, he has decided he still is young. The best way to describe this is with the cliché "young at heart": that is how the speaker feels, regardless of his true age.