Found the answer...Valerie was 14 in 1971, Julie was 19 when she disappeared in 75, and Lisa was 20 when she disappeared in 76.
So let's for a minute say that these three girls were killed by the same man and never caught and is also responsible for Elizabeth and Lryic.
This is really just speculating, but if the killer of those three girls was their age at the time, especially the 14 y/o then it's possible he was never considered as a suspect because of his age. Then the killer could have married in his early 20s and stopped killing. Possibly he has divorced in recent years and is single again and took up killing Elizabeth and Lyric.
This is really a long shot, but if that theory is correct then the killer would be 55-57. If he was the same age as those three girls when he killed them.
I'm not sold on this theory at all, but was trying to piece together something plausible after those three girls were mentioned up thread.