Matt and I had a similar problem when we were dating. My paternal relatives would have disapproved had he been older than me, but they were okay that he was the same age as my younger brother. On the other hand, my maternal relatives, being from the working class, insisted that I needed to look for someone older who had money and would be able to provide financial security. The thing is, I wasn't even looking for a boyfriend. All my life, my plans were to buy back our farm and raise sheep with maybe one farmhand. Matt wasn't looking for a girlfriend either. He and I just happened to meet on Facebook, and after arguing about football, we eventually hit it off.
It's not all Filipinas. I'd say it's more of a working class thing to want an older foreigner. A woman I know who married a Belgian twice her age said she was just so tired of working her fingers to the bone and not having any money to show for it after having paid for her younger siblings' school fees. Another reason I've seen is that the men in our working class really have that much less respect for the women. When Matt and I attended a family planning seminar that was required for the marriage license, we were one of only two couples out of 30 that didn't already have kids or who weren't getting a shotgun wedding because the woman was pregnant. As the nurse talked about contraception and the importance of spacing out pregnancies, the men in the room started bragging that they'd just cheat on their wives. I was dumbstruck that not a single woman defended herself from the public humiliation. In fact, the women all just meekly sat there staring down at their baby bumps and looking like trapped rabbits. Matt had no idea what was going on (it was all in Tagalog), but he said he could feel the fury practically emanating from me. How could I not be furious? The men proudly announced that they WOULD cheat because it was their nature as men, and their wives couldn't do a thing about it. The consensus among the men was that if the women confronted the men about the cheating, or cheated in retaliation, then they'd better not come home at all. And that was only one of the many times I met working class Filipinos who truly believed that cheating was a man's right. If you were a woman who felt that powerless financially or emotionally, would you not prefer an older foreigner too?
Something many working class mothers "jokingly" teach their daughters is to look for a man who is matandang mayamang madaling mamatay (aka the three M's). Roughly translated, it means a man who is old, rich, and decrepit (read: ready to die and bequeath everything to you). As you can see in aprilmae's example, many people do seriously want their daughters to marry old foreigners. In my maternal grandmother's case, she was also quite enamoured with pale skin so she added that to the list "for the improvement of the species". I felt like a breeding dog whenever she told me that. I was in the 6th grade (year 7) when she first told me to stop my stupid fantasizing about buying back our farm and to look for an old white man who would provide for me. When she saw Matt while I was on Skype, she just told me to ask Matt if he had any older relatives who'd be interested in me. Fortunately, that was all said in Bisaya.
Having said all that, being open to marrying an older foreigner isn't the same as actually wanting to marry an older foreigner. You may be hard-pressed to find a middle-class Filipina who wants to marry an old foreigner, but I'm sure there are lots who are at least open to it if they're approached by the right man.