Centres who have put up more or similar PPG aged 23-26 to Giroux in the last 20 years (Italics indicates in an era roughly over 6GPG average over the 4 seasons, now scoring is at ~5.3-5.4 goals per game.):
Crosby: 1.47 PPG
Fedorov: 1.37 PPG
Sakic: 1.29 PPG
Thornton: 1.27 PPG
Lindros: 1.25 PPG
Forsberg: 1.23 PPG
Turgeon: 1.20 PPG
Malkin: 1.19 PPG
Roenick: 1.14 PPG
Modano: 1.08 PPG
Spezza: 1.08 PPG
Zhamnov: 1.07 PPG
Giroux: 1.05 PPG
Sundin: 1.04 PPG
Lecavalier: 1.02 PPG
Weight: 1.02 PPG
Allison: 1.00 PPG
Getzlaf: 0.99 PPG
Richards: 0.98 PPG
Yashin: 0.98 PPG
Zetterberg: 0.98 PPG
Staal: 0.96 PPG
Backstrom: 0.95 PPG
Brind'Amour: 0.95 PPG
Toews: 0.95 PPG
Kopitar: 0.91 PPG
Gomez: 0.86 PPG
H. Sedin: 0.83 PPG
Marleau: 0.82 PPG
If you era adjusted that I think the only players Giroux is behind are: Crosby, Fedorov, Sakic, Thornton, Lindros, Forsberg and Malkin. I briefly looked over the era adjusted individual years and they are the only players with clearly better 4 seasons offensively between that age. Turgeon, Modano, Sundin, Spezza and Roenick are just behind him at a brief glance.
A lot of the players around and behind Giroux on that list were or are still 1st line centres in their mid 30's.
Sundin was still a 1st liner aged 36.
Brind'Amour was still a 1st liner aged 36.
Modano was still a 1st liner aged 35.
Weight was still a 1st liner aged 34.
Roenick was still a 1st liner aged 34.
Marleau is still a 1st liner aged 34.
Zetterberg is still a 1st liner aged 33.
Zhamnov was still a 1st liner aged 33.
Sedin is still a 1st liner aged 33.
Yashin was still a 1st liner aged 33.
Spezza is still as 1st liner aged 31.
Turgeon was still a 1st liner aged 31.
Lecavalier was still a 1st liner aged 31.
Gomez was still a 1st liner aged 30.
Allison was still a 1st liner aged 30.
They are the players ~immediately behind Giroux in scoring age 23-26 over the last 20 years. (adjusted very unscientifically.)
The players ahead:
Sakic was still a 1st liner aged 39.
Fedorov was still a 1st liner aged 35.
Thornton is still a 1st liner aged 35.
Forsberg was still a 1st liner aged 34.
Lindros was still a 1st liner aged 30.
That is 5/19 who were not 1st liner by their age 33 season. (Spezza is still unknown.)
However jump to 34 and it is 7/17 who were not 1st liners. (Sedin, Zetterberg and Spezza unknown.)
35 and it is 10/16 who were not 1st liners. (Sedin, Marleau, Zetterberg and Spezza unknown.)
The odds are in Girouxs favour until he is ~34... after that he will very likely no longer be a 1st line player.