To Age Single Plants, select By Singlular Planting..

Select a single plant or multiple instances that have been created using Place Single Planting.

Pick Finish when selection is complete.

Selected planting is displayed with columns for Grow Years, Height and Spread. Note that if any given plant is part of a hedge then the actual model size of that plant instance will be restricted to the specified maximum maintenance height/spread.

As the Years slider-bar is moved to the right, the number of years to Grow the planting increases and the grow Years, Height and Spread fields are automatically updated. Plant height and spread values increase up to the ultimate dimensions based on the number of years plants/species take to reach maturity. 

Years

Move the slider-bar to define the required years to automatically update the planting height and spread.

Pick Grow.