For those wondering about age as a factor for top D men, you should fiddle around with this website.
https://www.quanthockey.com/nhl/play...l-players.html
This should give some indication of comparable of D man performance age by age. Lots have played well, past the age of 34 but only a few from this era of hockey (2012 lock out and beyond) Notable mentions are Chara, Beauchemin, Markov, Streit, Timonen, and Boyle. Given those players performances after the age of 34, it should contextualize what we can expect from Gio in the next 4 seasons.
Edit:
To contextualize which D men over the age of 34 remain in the league ( have contracts) here is a list of them. Zdeno Chara (41) Brooks Orpik (37) Niklas Kronwall (37) Ron Hainsey (37) Deryk Engelland (36) Andy Greene (35) Dan Hamhuis (35) Duncan Keith (35). In total 8 D-men are entering the 2017-18 season with contracts.
https://www.capfriendly.com/browse/a...ge/all/defense