In the final season (season 7), JT mentions that Lily is 6-years-old. In the same episode, Carol tells Dana that that she wants Dana to stop fighting with JT because they've all been living together for 7 years. In other words, that is one year for every season. However, mathematically that means in order for Lily to be 6-years-old, she would have been born in season 2 but Carol actually didn't give birth to Lily until the finale of season 4.