I have in my notes that the man Peter heals is 40 years old, but I don't know how I got that information. I like to think of this through the man's eyes. He's asking for money at the temple when two men who he evidently doesn't know come up to him. He's thinking they are going to give him money. Peter says he doesn't have money, but what he does have is much more valuable and heals the man. The man is so happy, he's walking and jumping around, shouting his joy over what the Lord has done. I would think he's saying something like "I can't believe God did this, I can't believe God did this!" As he keeps hugging Peter and John, all the people come running up. They've seen this crippled man at the temple every day and know who he is and can't believe what they see.
Peter tells the people they shouldn't be so amazed. Who did this was the Savior, the same person they denied and killed. The same being who worked all the miracles in their sight, is continuing to do the same now. Then Peter says something interesting. He says, "I know that through ignorance ye had done this..." meaning they didn't realize what they were doing when they chose him over a criminal to be crucified when they could have set him free. He tells them that atonement of the Savior is in their scriptures. It was all prophesied, and now they should change and believe so their lives can be refreshed. I like that word refreshed which I think in this case means renewed. When the Lord renews our lives, he does it in the way that things will be better in the most meaningful way for us. It doesn't always coincide with what we want, but when all is said and done, we will admit that his way was the best because he knows us better than anyone.
Verse 21 is a great scripture telling of the restoration of the gospel. He says the heavens will receive the Savior until the times of restitution of all things. You can't restore something that is already there. He also says that the restoration was spoken of by all the prophets since the world began. We don't have those verses in our Old Testament today.