Quoting: LikeABoss
Huberdeau fell off a cliff when he couldn't play with Barkov anymore, not because he turned 29.
At the same point in their careers, Marner has 26 more goals and 115 more assists that Huberdeau.
Just a fun side fact, Barkov started playing 1 year after Huberdeau, and Barkov scored 181 goals by the time Huberdeau was 27. So they came up at similar times in the league together like Mathews and Mitch. The fun part is Mathews has scored nearly double what Barkov did... Just saying that 115 more assists seems a bit easier to come by when there's 187 more goals to potentially be assisted.
Just compare them to when Marner didn't play with Matthews much (2016-2020):
Marner (19-22) 300GP 83G 208A 291PTS (only 57 of those points are from goals scored or assisted by Matthews, just under 20%)
Huberdeau (23-26) 264GP 90G 175A 265PTS
Huberdeau only has slightly better production with 4-years of experience on him.
Marner in his rookie season, was playing with Tyler Bozak (and JVR) and had 61 pts, none of which Matthews had a part in. I don't recall who was his C in his second year, might have been Bozak still or Kadri, but Matthews and Marner connected for only 4/69 pts. Years 3 and 4 was mainly Tavares (think he played a decent bit with Matthews in 4). Matthews and Marner connected for 15/93 pts and 38/67PTS. Barkov had broken out a year before Marner's rookie season, and by time, Tavares was playing in Toronto, he too was an elite player. Huberdeau's production did improve as Barkov's did but it's not like Huberdeau wasn't producing all by himself with little to no talent on his team. It's no different than Marchand-Bergeron-Pastrnak all have drastic increase in their production around the same time.