In Season 3, when Ryan is with Sadie in the pool house, Seth finds the door locked when he tries to enter and states "when did this door ever have a lock." Ryan mentions he added it for privacy. In Season 1, when Hailey has the huge New Years party, she locks Ryan and Seth in the pool house, so they won't spoil her fun.
In Season 3, when Ryan was seeing Sadie and was unsure about asking Marisa to come to his birthday party, the calendar on the Cooper's wall had the number 19 circled as Ryan's birthday and the next day was indicated as the first day of Spring which would be in March. In the last season, Ryan is perusing a scrapbook put together by Taylor, which had included their hospital bracelets from when they both were in a coma. Ryan's bracelet showed the date 09-05-86.
It is established and shown throughout season 4 that Kaitlin is able to drive. But she is also 15. Californians can't get their driver's license until they are 16.
Throughout the series, it's clear that the actors/actresses portraying teenagers are actually in their 20s+. This is done to get around child labor laws, that won't allow teenagers to work full time on a set due to interfering with their real-life schooling. Plus casting adults adds to the benefit of better acting from the actors/actresses due to having more experience plus the producers are able to sexualize the characters without consequences. Lastly, it gives the general viewing audience who are teenagers (or young people) someone they can look up to.
In season 2 Kaitlyn is mentioned of living in the new house and when she returns she said that she went to the house that was next door to the Cohen's and some weird guy answered the door. She should have known that they moved if she was still living there at the start of season 2.
Correction: she never says she went to the house next door. She even mentions that it was Caleb's house, so she means the house they moved into when Julie married Caleb.
Theresa's story line makes no sense. She is supposed to be the same age as Ryan, as they attended school together, went to dances together, etc. Yet when Ryan is a sophomore in high school, Theresa gets engaged and plans a wedding for that summer.
In the pilot episode Ryan says he is 17. Each season was a year because of a Chrismukkah episode per season. Ryan doesn't turn 18 until late season 3 meaning he was either 17 for 3 years or he was retconned to be 15 in the pilot.