It depends. At the end of the day the people making the judgment on whether you'll be a good fit are only human. No two people will react the same way to the same set of experiences (including job hopping), so rather than worry about what constitutes job hopping, try to think about your situation and what the company is looking for.
Why Companies Avoid Job Hoppers
There are three main reasons companies avoid job hoppers:
- It takes time to find replacements
- It takes money to train replacements
- Having one foot out the door hurts productivity
Let's say I want to become an astronaut. Now I'm an awesome pilot. I have the required brains, and guts, and an easy-going personality. I am the ideal candidate to become an astronaut. So NASA invests millions and millions of dollars on my training so that I can go up in to space. They have me all set up for a mission when...
Now this is a worst-case scenario because of the incredibly high cost involved, but most jobs have the same issue to some extent. It is hard to find good candidates, it takes time to get new employees up to speed, and completing the job requires focusing on what needs to be done (if an astronaut is mulling becoming a US senator while training for a mission, the fundraising will probably get in the way of the training).
The general idea is, you don't want to appear to be a job hopper because it costs the company money.
Not All Jobs Require Investment
On the other hand, if I'm working in a call center reading scripts, there is far less training. I probably get a half-day seminar on how not to make death threats to customers or sexually harass my coworkers, and then get shoved in a call bank. Turnover could be 50% per year because the work sucks, and as a result they focus less on training, and more on making sure that any warm body can do the job without it.
If you quit from a job in a call center you're going to be viewed very differently than if you quit your job as an astronaut because the catered lunches sucked.
Think About the Company's Perspective
If I quit my job at a call center to go to another call center, they may look at the fact that I quit the last one after three months and say, "Sorry, we don't expect you to be here forever, but it's clear you're not cut out for this work even mid-term."
Think about what your company is looking for, and what, if anything, leaving your last job earlier will say about your ability to fill the needs of the new company.
If your last job was doing tedious tasks, and your new job will include that, quitting early may send the wrong message. If the jobs don't have much overlap at all, and the reason for leaving wasn't related to whatever overlap there is, it won't be as significant.
The number of times you hop also matters. If you have three jobs right after school that lasted three months or less, you may have an issue, but if you have a single job you left early with a good reason, it's not going to hurt as much.
If you know there will likely be an objection, prepare for it. Come up with a good reason that despite whatever objection to your job experience there is, you can explain why your positives are better, and/or why the negative isn't as bad as the interviewer may think.
What's Youth Got to Do With It?
Many jobs targeted at younger applicants are less skill and training intensive than those for someone with 5-7 years of experience. While your youth may give you a bit of leeway in explaining the job switching, or in finding jobs that don't expect a long uninterrupted term of employment, depending on what job or field you are looking at it may have an impact regardless of age.
For instance, if a company is looking for someone to invest heavily in for the next 10 years, someone whose work experience consisted of 3 jobs which didn't last more than 6 months, the investment has a lower chance of panning out.
When you think of statistical significance, a larger pool of data has better predictive ability. So if someone has been skipping jobs for a decade, it's going to be more indicative than someone who has been doing it for two. In the same way, a few quick job hops won't matter so much if you stick around in the next job for a longer period. Just be aware that the more frequently you do it, the more weight it will have, regardless of age.