If you right click a sugar cane, the age will drop to 0
MC-223123
Right-clicking sugar canes sets their age to 0
Resolved
Duplicate
0
.. ..
(Unassigned)
Unconfirmed
(Unassigned)
1.16.5
Retrieved
Dupe of MC-70127
Essentially, the number on the debug screen will not change as it is intentionally not getting updated to reduce the amount of traffic that is sent from the server to the client (even in singleplayer, this is true).