Oil paintings are made with pigment bound in oil, worked onto canvas, board or a wooden panel. The paint sits on the surface rather than soaking in, so you can usually see the brushwork, and in places the thickness of the paint itself.
That is the thing an oil painting has that nothing else does. Stand close and you are looking at the actual marks somebody made, in the order they made them, on an afternoon a century or so ago. It exists once. Buying an older one also puts a real painting on the wall for the price of a decent print, which is why plenty of people who would not call themselves collectors end up with two or three.
Age shows in the surface. Varnish yellows as it ages, which warms the colours and can make a sky look browner than it was painted, and older paint develops craquelure, the fine web of hairline cracks that comes from the layers moving over decades. Neither is damage, and both are part of how you tell an old painting from a new one.
Subject matters more than signature for most buyers. Landscapes, harbours and coastal views are the ones people come looking for, usually somewhere they know, and a good unsigned view of a familiar stretch of coast will sell ahead of a signed painting of nowhere in particular.
Paintings come in a few at a time, so it is worth looking again if nothing here suits.
