Fluids age with both miles, time, and faster with short distances between infrequent starts. That’s why short haul driving warrants more frequent heavy duty service intervals - the oil sump collects moisture and raw fuel from blow-by that isn’t burned off and becomes corrosive, and some fluid additives need time at operating temperature to do their job. Metals exposed to engine coolant experience localized corrosion when there are long periods without circulation at operating temperature, etc. Brake fluid draws moisture from the air (hoses and polymers are somewhat moisture permeable and some gets past the seals) and the swept surfaces of brake calipers and cylinders corrode and pit if they sit too long. Even the AC should be operated frequently to avoid shaft seal damage and leaks. 6mo. intervals is not too often for a car that sits around, especially with winters or a moist climate - if you want to know for sure you can have a sample analyzed.