I love to swim but I am not sure if it is good on the body, do you know?
I love to swim too. Swimming is actually a low impact activity and compared to running is better on the body.
Do you work a lot of muscles? I know my body is normally tired when I am done.
Yes.  It's a great workout.  I suppose it's different for people as swimming relies on the natural buoyancy of the body.
Does it have any long term effects? Like I know running is bad on the knees long term.
No long-term effects that I know for sure, but since the human body is less dense than water, water supports your weight while swimming.  No negative impact to the body at all.
Is swimming great for all ages then?
Yes.  Swimming is consistently top rated for public recreational activities.  It's safe for all ages.
At what age do people generally learn to swim?
Actually, swimming can occur within weeks of birth.