1. Is it a real startup doing real/cutting edge technology work ( aka cybersecurity, data, etc )? Will you be working on cutting edge development work?
2. Are you young with little to no responsibility ( aka no wife, kids, etc )
3. Do you believe the startup has a shot?
4. Do you respect and get along with the founders. And do they respect and get along with you.
If all the above is yes, then you may not earn as much initially, but you will most definitely learn a lot more than at google and you may be able to retire in your 20s rather than 50s if everything works out. If not, then you will be able to leverage the skill you picked up to get another job easily. Caveat being you do real development work in a real software startup.
If you are a single digit employee at a startup with a good relationship with the founders, not only will learn a lot technically and technologically, you will also learn about angel investing, VC funding, etc. You might even get to tag along to see what the hubbub is all about.
If it is a real startup, then the risks are higher but the upside is also higher. With google, you will get stability and name recognition, but you are still ultimately a corporate grunt and have to deal with burnout. Keep in mind google has extremely high turnover rates ( I believe the median tenure for a google employee is only 1 year ).