How can I check when a subnet was created?
On CloudTrail, select Event history, choose Event name from the first drop-down, and CreateSubnet from the second. Then fill in your timeframe.
If you don't have CloudTrail enabled, you will probably be out of luck, as I don't see a timestamp field available via the EC2 API.
References:
https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-create-a-trail-using-the-console-first-time.html
https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-subnets.html