I would let my oldest do that with my youngest. At 13, my oldest was a certified babysitter and had her own cell phone. She adores little kids, and they adore her. She's very mature and responsible. Naturally, I would set guidelines for where they walked, for how long, etc.
However, it would very much depend on the personalities of the individuals involved. My youngest is an easy-going kid. They have a lot of fun together. My second child is almost 13, and I would not trust him to do such a babysitting cervice. He just doesn't have the instinct to avoid trouble, KWIM? The oldest would never take the little guy by the pool, but #2 wouldn't see the harm in it. So, I couldn't let #2 take his brother out for a walk.