Asking senior scientists for a subreview I am a junior postdoc (1st year) part of a program committee in an international conference. As a member I have been assigned with several papers that I would like to delegate to subreviewers.

I have in my network researchers who are also postdocs but, they could be regarded as senior. Is it rude, or weird, to ask them to subreview a paper? If so, should I try to find PhD students or other postdocs in my range? If not, to what extended may I ask for a subreview?
You can always ask and they can say "no". It's surely not rude and weird; people are asked to review stuff all the time. The probability is not too low that one or more of them will say "no", but you may be lucky. 

In my experience, more senior people are more likely to say "no", but of course if they say "yes" they will be the more competent reviewers. And some say "yes". 

Don't be worried about asking somebody something that they don't want to do. If you ask and they say "no", it won't reflect in any way negatively on you.