I am looking for a book I read in high school (around late 80's, early 90's). Its a young adult sci-fi book about teenagers who were born on the same day and in the same hospital. All are best friends who start to develop powers (from what I can remember) and the title had the word 'experiment' in it, I think. My searching has been unsuccessful!
You may possibly be thinking of John Wyndham's The Midwich Cuckoos.
It's about 31 boys and 30 girls who were all born on the same day and when they grow up, they slowly develop all kinds of powers, such as mind control.
This sounds like Experiment in Terror by Bernal C Payne. The cover matches user3351's description. I read this book dozens of times in high school in the early 90's.
I've actually been looking for this book for a long time, so thanks for asking the question.