The age distribution is a valid question, but tangential to the subject sample size. Increasing the number of subjects to 340 still won't resolve your problems

The age distribution is a valid question, but tangential to the subject sample size. Increasing the number of subjects to 340 still won't resolve your problems with the distribution. The statistical validity of the brain regions differentially lighting up holds even if the networks in the brain changed drastically with age. You can definitely argue the assumptions behind the study, but the methodology is pretty standard and has better-than-average rigor.