We went to Bilbao with our 4 month old baby.
This apartment was absolutely excellent. So close to anything that you would need! Old town was just on the door step, and you didn't need to walk for longer than 25 mins to get to any place of significance.
The apartment is on a hill, so you have to trudge up a little to get there, so as long as you have a average level of fitness you will fine! But it is a beautiful little apartment. All the pictures do it justice.
The host was so helpful with getting there from the airport. He gave us a few suggestions on to where to go for food and where to visit. He even came to find us when we were lost getting to the apartment in the first place.
Will definately be using this apartment the next trip we make to Bilbao