I don't know why they couldn't have just said that Barnabas was 35, he looked 35 at the youngest, and there's nothing wrong with being 35. It was quite common for the husband to be older than the wife then. Jonathan Frid certainly didn't look 25! It is much easier to play older than younger.
Barnabas would still be the same character if they had him be 35 or so, andit would have been much more believable too.
On a different subject, remember when Barnabas used some sort of magic to make it seem that Dr. Woodard was haunting Julia? He said that he learned this in Barbados. Then when the show went back to 1795 they wrote him to be all innocent. Not at all the same person he was in 1967. In 1795 he didn't even believe in witches.