The way I see it, you're asking the wrong question. When it comes to joining startups, the question isn't usually "is this the right time for me in my career". Instead, it's "am I the right kind of person to take/is this the right amount of risk for me".
Startups are not easily/well described in terms of career. They are better described in terms of risk.
There are loads of risks that you are taking on with a startup, including financial, personal life, health, legal and career ones. The potential rewards are commensurate - some succeed spectacularly, most fail, and some fail spectacularly. On average, the payoff is worth the effort. At a regular job, you'll see mediocre rewards, with the occasional above average and occasional below average spike. The top most rewards and worst failures are kept at a distance from you.
From a strict career perspective (do not discount the other components! think carefully of the financial, personal, health and other risks!), ask yourself whether:
- You are generally a confident, self starter, who is comfortable taking personal responsibility for the next stage in his professional development.
As the least experienced member in a startup team, you may end up in an awesome situation, with a great mentor who takes loads of care to nudge you along in a good direction. Most likely, you'll be dropped into a warzone, and find yourself doing grunt-work which may not be useful for anything specific once you're done (and which you may not even be able to describe!). That is, unless you have the chutzpah and self drive to learn how to contribute
- You are comfortable with failure
Most startups fail. Most likely, the startup you're joining will not succeed. Even if it does succeed, it will most likely (at least initially) fail you from a career perspective. Ask yourself if you are confident in the knowledge that, after the first one fails, you will come back stronger, learn your lessons and do it all over again. And again. And again.
- You know how to offset risk
I've listed some of the major career risks. From this perspective, knowing how to offset risk means, among other things, that you can, or have already, find your own mentors, outside of the startup. You know how to balance your work/life and take on side projects for your own development, outside of work. You know your field and have a good map of where you want to go, and have done the due diligence on the startup to make sure that what they want you to do will teach you skills which match up with your long term goals
Startups are popular now, and there is an unusual amount of support available for starting them and joining them. If they are the right thing for you, this is a good time to go for it. But don't let that be a major factor in your decision. You may be the right kind of person for them later on in life, or not at all - in which case, joining one right now is probably a bad idea.