Timeliness of information is critical for accurate decision making in data-driven systems. As large-scale systems based on Internet of Things (IoT) technologies start populating shop floors, their performance in terms of timeliness must be assessed. This work analyzes age of information (AoI) in IoT-based control systems, where independent control loops share a wireless network. Analytical expressions for AoI under two typical MAC schemes used in IoT systems: CSMA/CA and TDMA are given and validated by simulations. The significance of the results is illustrated in the context of a design example.