Assuming you are using an EBS-backed instance and aren't doing any fancy drive juggling, the best way to figure out the creation date of an instance is to look at the creation time of the drive's root volume. While the launch time of the instance will change every time you stop and start it, the creation time of an EBS volume is static.
Here's a quick script to find the creation time of the instance you're currently running from:
import boto
import subprocess
instance_id = subprocess.check_output(['curl', '-s', 'http://169.254.169.254/latest/meta-data/instance-id'])
conn = boto.connect_ec2()
root_device = conn.get_instance_attribute(instance_id, 'rootDeviceName')['rootDeviceName']
root_vol = conn.get_all_volumes(filters={"attachment.instance-id": instance_id, "attachment.device": root_device})[0]
print root_vol.create_time
Note that this requires the IAM Role of the instance to have ec2:DescribeInstanceAttribute
and ec2:DescribeVolumes
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