We have seized computers from a person accused of sexual assault of a child. There are numerous pictures of young girls which are lewd, etc.

How do you go about proving the age of the young girls in the pictures?

You can get there by calling a medical professional, usually a pediatrician, to give expert testimony about child development and to opine about the approximate age of the child in the image.
I've used Dr. Beth Nauert here in Austin in the past and she's great. Pretty much any doctor who deals extensively with children, and particularly child sex abuse victims, is more than qualified to render such an opinion.
If the images look like they may have been homemade by your suspect, then you obviously would want to try and find the victims, and there are lots of ways to go about that. Often juvenile officers or school police officers will recognize them. Also, we've found some by just digging through school yearbooks.

You also should read an earlier thread, in which there was a discussion of the effect of a Supreme Court case on this issue. Just go to Porn Case.

Dr. Nauert is great, but two other good experts to use to prove a child's age are Nancy Kellogg, M.D., who is medical director of Alamo Children's Advocacy Center in San Antonio, and further away, is Sharon Cooper, M.D. in Chapel Hill, N.C.
E-mail me for contact info on either of them. I went to a course called "Protecting Children Online" in Tuscon that dealt with child porn and Dr. Cooper taught the medical portion of the class. She and Dr. Kellogg have written an articles on this subject. Things to look out for: If you find an expert who tries to talk to you about the "Tanner Scale," get someone else, because use of the "Tanner Scale " to determine age is outmoded. Kids go through puberty much earlier now than they did when that method was developed in the 1960's (that's because of hormones in food, better nutrition, etc. Another factor to watch out for in determining age is malnutrition and drug abuse. They said at the class that a lot of the child pornographers will seek out street kids (runaways, drug addicts) to pose for photos. So sometimes, what looks like a prepubescent teen is really an 18 year old homeless crack addict. Limb proportionality is a big factor in determining age, as well as face shape. You can't just look for breast development and pubic hair.

Of course, we had a case where the defendant labeled his files, "sevenyearolds.jpg." That's a pretty good clue.

The Customs Service and National Center for Missing and Exploited Children (web page) have launched a database to catalog child pornography and identify victims in the wake of the recent Supreme Court decision on computer generated images. If you find that the stuff is homemade or can otherwise identify victims, it would be worth a call to the program to get them cataloged in case they were uploaded and now floating around the web.