I work on the dolphin project with four other people. It depends on which species you are talking about. Bottlenose dolphins may live for 50 years, but most only live for 25 years. Bottlenose dolphins only stay under a few minutes on average. It depends on what they are doing.
Wild dolphins are opportunistic feeders.
If food is available they eat because they don't know when the next meal will be. You tell the sex of a dolphin by looking at the position of the genital slit on the belly. You have to be close to do this and it is not easy to do. The male's genital slit is closer to its belly button than the female's. However, if you are looking at adult killer whales it is easy to tell males from females because the males have a huge dorsal fin.