This is correct. They were 15 and 12. The ages of 18 and 24 are clearly a mistake on that website.
Patricia was 12 years old when she went missing on 28 December 1956 - 3 days before her 13th birthday. Their bodies were not found until 23 January 1957, so any reporting done between 31 December and when they were found would have said she was 13 (as seen in her Websleuths thread title.
I have seen other similar cases in which ages are off by a year because of a birthday occurring shortly after someone went missing. Also, some accounts might have only birth year and death year, and this can lead to mis-stating the age.
Yes, one of the articles or podcasts mentioned Partricia had a birthday coming up. Her mom said she had already invited several of her friends to come to a party with birthday cake and soda.
It's a shame that LE wasted so much time pursuing the dishwasher as their main suspect. It should have seemed obvious that whomever killed the girls owned or had access to an automobile. You can't snatch teen and pre-teen girls off the street, assault and kill them, then dump their bodies miles away in the countryside without a car or truck.