I think Lord Sugar prefers younger people. I feel like they have often done better on the show. Many of the winners have been in their early to mid-twenties - the oldest was Tom, who was 31 at the time - and the youngest so far has been Leah from last year. Bianca and Mark are 25 and 24 respectively. He has been very positive about some of the young people who didn't win as well, such as Susan Ma. Also he seemed extremely passionate about Young Apprentice - more so than about the main programme really. I feel like he prefers fresh faces who have potential but haven't learned so much as to become set in their ways, so he is able to mould them a bit more.
By contrast, the older candidates have often not done as well. The oldest ever - Jaz - was fired first. Before last year, that record was held jointly by Matthew and Mani, who were both 39 at the time, and both of them were fired as early as the fifth week.