Length of resume in your 20's, it should be one page.
If you have a bit of experience (e.g. 5-9 years), then maybe put an "Early Career" section to show that you did stuff, then progressed. These would be 1-2 lines per job. This can help show that you know how to do stuff.
Do you want to do it? If your resume is a list of things that you don't want to do anymore, then either drop them, or change the wording.
Is it relevant? If you are in IT, then your HS job of mowing lawns for a few neighbors should drop. But, if you managed a swarm of HS students mowing 50 lawns, had an LLC, bought a riding mower, etc. then it might be relevant if you are going for a business job.
Is it known? If you have an item like an organization or conference or whatever that is not known in your field then it won't help. It also won't help if it is not impressive in some way (at least impressive vs. others who are applying for the same job).
Does it show that you will DO? Employers want you to show them can you can actually do something. They don't really care about your GPA; they don't care if you are a member of church such and such; they don't care if you were in some organization. They want to know, "What will you do for me?" and "How will you help my organization make money?" Use your resume to tell this.
Your resume should be one page. If you don't have enough stuff to fill that, then yes, put on older, less relevant things.
When I'm hiring someone, I want to know that they know how to work a job. Even if all you did at some place was fetch coffee and wash the boss' dog, you have some experience in an office, and understand office protocols, politics, etc. better than someone who has done nothing.
The other thing is that if you have had 2 or 20 short-term part-time jobs in HS and college, that's fine. People are still finding themselves and are finding where they might work. Contrary to another post, no many people can find a job in their career when they are 13. Many people are 30 and still don't know what they want to do - or they've done something they loved, but now need a change.
If you send me your actual resume (even 2-pages would be fine), I can be more specific. (No charge, I'm not a resume writer, just a manager and teacher.)