2d plot with modification of plotting symbol to indicate third dimension.

I would like to plot 3-dimensional data on a two-dimensional scatter-plot.
Is there a way I can automatically modify the plot symbol (e.g. changing size or color) to indicate the value of a third variable? E.g. How can I plot weight vs. age and indicate the value of muscle mass for each value weight-age pair by making the plot point proportional to the subject's muscle mass?