The point is that when making the comparison "Age" in itself means nothing, but rather time in the league.
Let's look at the timeline this way:
Roy Halladay:
Year 1: 14 IP, 1.93 ERA
Year 2: 149 IP, 3.92 ERA
Year 3: 67.2 IP, 10.64 ERA
Year 4: 105 IP, 3.16 ERA
Clay Bucholz:
Year 1: 22.2 IP, 1.59 ERA
Year 2: 76 IP, 6.75 ERA
Year 3: 92 IP, 4.21 ERA
Year 4: ???
What the article tries to ascertain is the similarities in career paths (i.e early succes, struggles, then a settiling down period) and tries to use the rising groundball rate as the reason for what later became sustained success for Halladay and could become success for Bucholz as well.