You can't find as such attribute called create_time for EC2 instance because only launch_time is available in it.
But yeah, you can use a Python code to know when was the volume created which in return gives you instance creation time, it gives the only volume attached while creating an instance
ec2 = boto3.resource('ec2', region_name='instance_region_name')
volume = ec2.Volume('vol-id')
print volume.create_time.strftime("%Y-%m-%d %H:%M:%S")
The alternative is using custom code. When you create instances with create_instances(), you can log the launch_time for given instance along with it's instance ID and name to some place like DynamoDB so that you can retrieve the "create times" whenever you want using the instance IDs.