Heart rate varies depending on age. / Customs vary depending on the age.

I’d appreciate it if someone would answer my question. Thanks in advance.

Concerning the following 2 sentences------

A: Heart rate varies depending on age.

B: Customs vary depending on the age. (Here “age” means the time we live in.)

Why in A no article is used, while in B a definite article is added?

age - non-specific, as it is talking about age in general not one person in particular.
the age we live in - specific. When talking about the modern age, the middle ages, the neolithic age, etc, the article is always required.

-Thank you Beeesneees. I see your idea.
Another question please! I’d like you to check my explanation about the following 2 (C and D) is right or not.

C: The charges vary depending on weight.

D: There may be additional charges depending on the weight.

I think “weight” in C means weight in general, so an article isn’t needed. C is said generally. On the other hand, “the weight” in D means specific weight, such as the weight of a customer’s parcel that is going to handed over to a post office clerk.
Is my explanation right?

Sounds reasonable to me.

Thank you Beeesneees.