Student or Learner
Here are two sentences that I am not sure which one is the best to compliment someone on how they keep themselves young and energetic:
1. You look younger than your age.
2. Your face does not match your age.
I do not know what other native speakers think, but I, for one,
do not think that the sentence above would be understood
Let's say someone is 50 years old. Your sentence could be
Your age of 50 does not match your age of 50.
I believe that you wish to tell someone that s/he looks younger than
his/her age. I think that you will just have to say it that way.
Oher ways are ambiguous:
You don't look 30. = You look 19/ You look 50.
You don't look your age. = You are 19, but you look 14; You are 19, but
you look 30.
English has many strengths. Like all languages, it also has many
weaknesses. Ambiguous sentences are one of them. Of course,
sometimes ambiguity is a good thing. But probably not when you are
discussing someone's age.
***** NOT a teacher *****
In general, in the US, reference to age (especially for women) is simply not welcome.
I'm 44. If someone said "Oh, you have so much energy for someone your age" I would wonder if they though people who were 45 must need a cane or a walker. There are few things you could say using the word "age" that would be 100% safe.
If you want to be complimentary, just say something like "You have great skin" or "Wow, you have so much more energy than I do."
Yes, we like to be told we look younger, but you have to be so, so careful how you do it.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.