If we keep the comma in this sentence, can we consider the second part to be a non-restrictive relative clause?
***** NOT A TEACHER
*****
Hello, Diamondcutter:
In my OPINION, the answer is NO.
That is to say, "which you can learn even at an early age" is not a proper non-restrictive clause in "Helping others is a habit, which you can learn even at an early age."
As the other posters said, one needs to say "Helping others is a habit that / which you can learn even at an early age." (a restrictive clause)
*****
If you want "which you can learn even at an early age" to serve as a non-restrictive clause, I think that it will be necessary to tweak that sentence:
"The habit of helping others needs to more widely practiced."
"You can learn that habit even at an early age."
"The habit of helping others
, which you can learn even at an early age, needs to be more widely practiced."
James