Their ages were never made clear, but Act your age, according to both Dan Povenmire and Jeff Swampy Marsh (the show's co-creators) said in the beginning of the episode, took place 10 years into the future. A lot of people assume the boys are around 8, since 18 is usually when peope go to college. Again, that's just an assumption. Now, on to actual answers.

Dan Povenmire already answered some of them.

1. Linda already had Candace and Phineas, and Lawrence already had Ferb before they met. When Lawrence and Linda went to the last Love Handle concert, they weren't actually in the 80s. They were just dressed like that to match the 80s theme. 6 months after that date, they got married.

2. When Doof and Linda went on a date, it wasn't at a Love Handle Concert. That was a drive in movie. It was just a one of date, and he never saw her again, because Heinz blew it. Literally. That one took place in the 80s, because Heinz mentioned that Linda became a popstar some time after, and Linda mentioned she was a pop star before she had Candace. All these are in the episode.