Does your child like playing with a younger (2.5 yr) old child?
In general, but being completely dependent on the personality of the older child, children 4-6 get frustrated playing with a child that much younger. After about 6-7, the older child would most likely see the younger child as being younger enough that he is not a playmate, but more of someone to take care of and entertain, like a mama does. Which can work just fine as long as the adult is supervising pretty well.
The difference in skills in playing are so much more profound when they are that young. The 4-6 year olds are moving out of the chaos stage - where blocks were basically great for building towers and knocking down, for example, to now being useful for building roads and structures for cars and dolls, with rules and scripts and all that. Its hard to be at the age where you are just beginning to enjoy that kind of play but your playmate can't grasp those concepts yet.
When my younger kid was 2.5 and his sister was 5.5, they did play together quite a bit, but the 5 year old would get frustrated with her brother a lot, too. I could see a 4 or 5 year old having some fun playing with an easy-going 2.5 year old who could talk well, but I don't think an older kid would enjoy it. (Except for short periods in an "oh, isn't he cute!" sort of way.)
Kids that are used to mixed aged environments would have a lot of practice and scaffolding in negotiating relationships and shared space with kids who are younger. (A kid in a Montessori environment or other mixed age school).
I watch several kids with my 5 year old in tow: an 18 month old, a 3.5 year old and 6.5 year old mostly (and sometimes all of them at once). Ds mostly loves it. Occasionally I take the toddler out of the room to do something with me when all the kids are over because they can get a bit wild, but mostly we all hang together.
If the older kid is the child of the babysitter I would suspect they are used to be being around younger kids and understand that this is their parent's job. They may even feel like it is their own job. I know ds always says "are *we* babysitting today?" He takes a lot of ownership and pride in *our* work.
in a situation like this - i would not write off a mother with a 4 or 5 year old just coz of statistics.
i would interview all of them with their children and see which one fits your family's needs.
at 5 my dd could play with some younger kids and not with some. depends on their personality.
I am interviewing all the moms because I definitely want to see what works. I specially want to support nannies who become moms and find their job opportunities limited. It's a win win I feel ( with some effort on both sides). I asked on this forum just to get a perspective since I have no clue about older kids.