That's one hell of a question. But, every old birds in the microstock industry will tell you that it is never too late. However, 10 times tougher you must expect (with Yoda voice).
Contributors used to sell hundreds of downloads for a simple set of 16 vector icons during 2008 era. One such simple set of icons can generate you thousands of dollars over a short period of time. Today, a big set of 100 - 500 vector of icons can only generate you $10 dollar a month. Though, this is just my hypothesis, but I believe many can agree and more would agree soon.
Too many people contributing too many high quality works causes the meltdown. The supply is overshadowing the demands. Do you want to know who you are competing with? Take a look at this.
Searching for the keyword "icon set" at Shutterstock.
Searching for the keyword "silhouette" at Shutterstock.
Searching for the keyword "animal cartoon" at Shutterstock.
Searching for the keyword "labels" at Shutterstock.
Are you up to the challenge? Can you produce such quality and quantity? If you think you do, then it is not too late for you. And all is not lost. There are contributors who are still be able generate a comfortable side income from this with less quality works. You must be able to hit the right niche and continue to be committed to it.
If you really want to give it a try, then I recommend you to try the below 3 agencies first due higher volume of downloads and acceptance rate.
Shutterstock (must try first)
The golden era of microstock is definitely over, but that doesn't mean you cannot create your own golden era if you start now.