I don't believe that sweepstakes discriminate against the elderly. After all, sweepstakes are free to set age restrictions if they want to reach a specific age group. There is nothing stopping them from restricting entries to people between 18 and 35 years of age, for example. So it wouldn't make sense for them to allow entries from people they don't want to reach.
It is a good idea, however, to evaluate the sweepstakes that you enter to be sure you are able to participate in the activities offered. For example, if a sweepstakes prize is a trip to Hawaii to compete in a marathon, then you should avoid entering if you are not up to running a marathon. Of course, this is true of people of any age. Find out why it pays to be picky when entering sweepstakes.