I don't think you can say definitely an age or a day, however my extensive research points to a conclusion which I think few can doubt:
GMs are at their strongest during the period in which they are alive.
You are welcome. I am always willing to help with such analysis, because I owe it to my fellow chessplayers.
I thought narrowing it down to a day of the week was already a major achievement (compared to all the other wishy washy studies) and was all set to celebrate