In general, the research does not support good long-term outcomes for retentions and redshirting (individual cases, of course, may vary), especially for boys, who have a significantly increased likelihood of high school dropout, compared with non-retained age/ability-matched peers. Granted, the research is focused on retentions for academic or social maturity reasons, not GT students. The one place where being old-for-grade does appear to be somewhat advantageous is in competitive sports, where being more skilled or stronger because you are older appears to translate into being perceived as more talented, and thus more worthy of playing time and coaching attention. This really only works if your late birthday is after August 1st, though, because there are fairly strict birthdate cutoffs for competitive team sports in most communities.
Agree with st pauli girl that I wouldn't delay entry for maturity reasons. I would go further, and say that an age-eligible child not being within the range of maturity for kindergarten-age students is an even stronger argument for being in school, in order to access services for social skills development. Especially considering the diversity of social-emotional skills in primary-age children.
