In F3 debug details, the wrong age is shown for cactus and sugar canes. It is almost always 0 and sometimes 1.
Edit: After some more testing, I found out that the age updates, if you go far enough away (to unload chunks?) and come back. F3+A does not help.
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Works fine if the tick speed is set to normal.
As far as I know, age
should increment when the block gets a tick, by random values from 0 to 15, but it does not.
No, it always increases by one each time it is ticked.
After some more testing, I found out that the age updates, if you go far enough away (to unload chunks?) and come back. F3+A does not help.

Confirmed for 14w30c
Plant 1 sugar cane
/gamerule randomTickSpeed 3000
Look with debug window (F3) at the sugarcane (its age stays at 0)
Rightclick the sugar cane
Now it shows the correct age
Same with cactus but NOT with wheat
It happens at any tick speed.

Various debug screen fields aren't updated in the client unless the block receives an update, or the entire chunk is resent, such as when you unload and then reload the chunk. The debug screen is primarily intended for Mojang to use while developing the game, and not for the players, so this is unlikely to change unless it would serve a development purpose.

So it works with wheat because it changes its data value and texture?

Yes. The change in texture requires that the client's information about the block be updated. They've tried to limit unnecessary updates to minimize network traffic, and thus reduce lag.

Alright thank you 🙂