No Democrat could have defeated Reagan in 1984, but Glenn would have made it a much more competitive race.
The year for Glenn to run would have been either 1988 or 1976, though he may have been considered too inexperienced yet in 1976 since he had only been in the Senate for two years (national celebrity notwithstanding).
1976 would have been a good year for any competent Democrat, but Glenn would be basically running as a celebrity with less than 1/3 of a Senate term, not to mention a Republican would take his Senate seat. His year would have definitely been 1988, where he would have won if he chose a more "mainstream" running mate, like Bentsen