I'm not an escort so I'm attacking this topic strictly from a dating stand point. I do believe my preference and reasons would be quite similar if I were to be an escort though. Anyways, for me, it's first and foremost their level of maturity, followed by their taste and ability to afford some of life's luxuries.
1. At 21, I find most guys my age to be very irresponsible, not very straight forward with their intentions (Do you want gf, or fwb? Stop playing games already), and our interests typically don't overlap.
2. I grew up in a cultured environment. I love going to plays at the theatre, I live for dining at upscale restaurants, and I enjoy traveling. I also have the funds to do it. Guys my age seem to lack an interest in such things, and also don't have enough discretionary income to engage in such activities.... I can pay my way and even foot the bill if the guy lacks the necessary funds, but a lack of interest and complaining in particular pisses me off.
Those are the two main reasons. My last reason is a rather shallow "because older men are hot!". Give me Hugh Laurie or George Clooney, and I'll be one happy gal. Sean Connery? I'd probably die from all the excitement (I have an unhealthy, pre-teen originating obsession with him). Hell, even Boris Kodjoe could get it. There's just something about a grown and sexy gentleman, especially one in a suit who's witty and commands a certain level of respect that gets me moist with excitement.
Hot 20-30 y.o. just seem like cute little play things to me. This may be partly because my experiences with them have led me to believe they are very fickle and into playing games, but I've always been smitten by the 35-40+ demographic.