Combination skin is probably the norm, in the sense that some areas of the skin produce more sebum that others. The so called “T-zone”, includes the forehead, nose, and chin, and tends to be more oily than the rest of the face. As people age, sebum secretion decreases, even in the T-zone. This is the main reason why the incidence of acne also decreases as we age, as the sebum fatty acids is what keeps acne bacteria going.
What to do then, if you have combination skin? Just use the same products all over, and reserve some T-zone serum for the area of the forehead, nose and chin, as needed.
Remember that oily skin is “bad” only in the sense that more sebum means more sustenance for the acne bacteria, but in the long run oily skin is likely to age more slowly.
If you are looking for detailed instructions on how to deal with combination skin, the idea is to be sensible, and look after your skin by not mistreating it, like exfoliating it to death, or exposing it to the sun or heat for too long. Don’t use alcoholic toners (which shouldn’t exist) but use instead a mild but effective cleanser like the gentle cream cleanser or the daily cleansing gel.
Use a non-greasy sunscreen but still limit exposure to the sun as much as you can, because sunscreen will not block all the effects of UV.