listen, the age does not matter as much unless we are talking extremes. it is very difficult to get a new health insurance for a person above 60-65 but most companies will continue the insurance if you signed up years before.
pre-existing conditions will not be covered, but normally there is no medical checkup.
expect to pay up to 3000 pesos a month for a family of four, depending on a coverage. traditionally plans have no limit on emergency and limits on other hospital services (up to few thousand pesos a day). coverage of doctors's visits are about 80%. coverage of the purchase of medicine about 80% too, up to a certain value a year (minimum 3000 pesos, for better plans 5-8 thousand).
you can tweak the details: limits of meds and hospitals, extension of the coverage (dental? maternity? eyes?).
you can add people to your plan as you please: new kids, parents, in laws, domestics and so. you can also kick them out.