Which is very important

[Rachel Adams]

Can I use "which" to mean that "sleep pattern" or getting regular sleep is important at her age?

She doesn't have a healthy lifestyle and her sleep pattern isn't normal which is very important for her age.

 

[Tarheel]

Two things. One, you're trying to put too much into one sentence. Two, you're asking "which" to do too much.

Perhaps:

At her age she should be getting plenty of sleep. Unfortunately, she's not getting the sleep she needs.

 

[emsr2d2]

Using "which" like that suggests that having an abnormal sleep pattern is very important at her age!

 

[Barque]

She doesn't have a healthy lifestyle and her sleep pattern isn't normal which is very important for her age.

She doesn't have a healthy lifestyle, or a regular sleep pattern which is very important at her age.
If you want to say they're both important at her age: She doesn't have a healthy lifestyle or a regular sleep pattern which are [both] very important at her age.

Note the change from "for her age" to "at her age". "For her age" could work in a different context.