In the present study, the effect of sludge age was evaluated for simultaneous sludge reduction and nitrogen removal in an oxic-settling-anoxic (OSA) system fed with real domestic wastewater. Three laboratory-scale systems utilizing aerobic and anoxic zones and step feeding regime were operated for sludge age of 13, 17 and 20 days in the main reactors. A significant influence of sludge age on the sludge reduction was observed compared to conventional activated sludge systems (CAS). The greater corresponding sludge reduction was achieved as 58% operated at interchange ratio of 7.7% (1/13) in the side-stream reactor, while others revealed 37% and 35%, where interchange ratios were 5.9 (1/17) and 5.0% (1/20), respectively. In both CAS and OSA systems, high removal efficiencies of organic matter and nitrogen were achieved using real domestic wastewater. The results indicate that intermittently aerated OSA systems could accomplish less sludge production and higher nitrogen removal (up to 85%) simultaneously. Thus, it is suggested that interchange ratio of around 8% is more optimized level, which is a little lower than that of proposed and applied in most studies in the literature, which would possibly be more cost-effective.