Age‐related changes are usually overlooked in perennial grass research; when they are considered it is usually as a change in plant size (e.g., biomass). Whether other physiological or developmental aspects change as stands age, and how those aspects may impact long‐term stand dynamics, remains unclear. Conventional experimental designs study a single stand over multiple growing seasons and thereby confound age‐related changes with growing season conditions. Here we used a staggered‐start experimental design with three repeated planting years over two growing seasons to isolate growing season effects. We studied changes in Miscanthus × giganteus phenology during its yield‐building stage (first 3 years) and estimated age, growing season and nitrogen (N) effects on development using nonlinear regression parameters. Stand age clearly changed plant growth; faster developmental rates were usually seen in 1‐year‐old stands (young), but because 2‐ and 3‐year‐old stands (mature) emerged 3 months earlier than newly planted stands they produced 30% more stems with 30%–60% more leaves. Nitrogen fertilization modulated some age‐related phenological changes. Fertilized 2‐year‐old stands reached similar stem densities as unfertilized 3‐year‐old stands and had fewer number of senesced leaves like 1‐year‐old stands. In addition, N fertilization had no effect on young M. × giganteus, but extended mature stands’ growing season more than 2 weeks by hastening emergence and delaying senescence. It also delayed flowering regardless of stand age. Our results suggest that, along with changes in size, M. × giganteus stands showed shifts in developmental strategies: young stands emerged later and developed faster, while mature stands grew for longer but more slowly. In temperate regions, where hard frost events are likely to interrupt development in late autumn, rapid early development is critical to plant survival. Nonlinear regression parameter differences proved effective in identifying phenological shifts.