If 40% of those university graduates are still overqualified by their mid-thirties, they've already been typecast by their experience in the 25-35 range.
That's certainly a problem with the data provided--it bundles together the fresh-out-of-school 25-year-olds with the decade-plus-in-the-workforce 34-year-olds. There's a lack of resolution. It could be that 40% of 25-year-olds and 40% of 34-year-olds are "overqualified". Or it could be that 60% in the 25-29 age group are overqualified, and just 20% of the 30-34 bracket.
Actually, that brings to mind another confounder to the interpretation of these data. As more young people get more years of formal education (3-year college diploma to 4- or 5-year bachelor's degree to 7-year bachelor-plus-master's degree) they enter the workforce later. A 25-year-old with a high school diploma might have been working for 7 years (and is also more likely to be working in a job for which they are not "overqualified" by their lower level of formal educational attainment). A 25-year-old with a master's degree might have graduated this summer and could still be job-hunting.