There are pros and cons for both young and old yayas. It depends on what qualities you are looking for in a yaya and what you are more comfortable with.
Young yayas are more physically fit, hence, more able to watch precocious and hyperactive tots; older yayas might not be agile enough to run after highly energetic 5-year-olds. Young yayas may be easier to train, given their minimal experience with childcare. But some young yayas are not ready for caring for a child, if not completely clueless about it. Older yayas, on the other hand, may have fixed notions about child-rearing that are passé and do not coincide with your own, which may cause conflict. Worse, some older yayas may be too serious or stricter in raising kids. However, some older yayas may also be gentler, kinder, and mellower in disciplining your child. Moreover, younger yayas do have a tendency to leave on a whim (perhaps to pursue a more “exciting” job) than older yayas, who tend to stay with a family longer for sentimental reasons.