I'm 49 and have been researching this very same thing lately. It's seems that as we age, our shbg rises. According to some studies, this rise in shbg renders a portion of the exogenous testosterone taken to be made biologically unavailable. But then there are studies out there that claim that even though testosterone molecules have bonded to shbg, the body can still free this testosterone if needed.
As for me, due to my results as of late, I am beginning to lean toward the hormone remaining unavailable once bonded. I do not have enough testing results at this time to prove this either way, but have decided that this is possibly something we need to worry about while using aas as we age.
One interesting study about shbg levels claimed that supplementation with bromine can assist in substantially lowering our shbg levels. I haven't been able to source bromine locally so I am planning to order some soon and see if it does actually lower shbg levels on testing and help to improve my results achieved while on cycle.