The age of a plant determines its size at planting time. You can increase or decrease the age of a plant by making a change on the plant’s Plant Info property page.
To change the age of a plant:
Click on the plant whose planting age you want to change.
Right-click and select Properties, or select Edit > Modify Elements > Properties.
In the Plants dialog, select the Plant Info tab.
Move the Planting Age slider left or right until the desired planting age is displayed. Planting age is measured in years.
Click OK.