They mean slightly older girls. They are objectively better matches because they are more established in terms of parnasah, more equipped to deal with the realities of life etc etc.
To the best of my knowledge there are only two objective benefit to marrying a slightly older girl vs a girl slightly younger.
1. Almana Factor-women tend to live longer than men and thus if the wife is younger (and especially if she is significantly younger) she will in all likelihood be a almana at the end of her life. Sad but true. visit any assisted living or nursing home at she the female/male ratio.
2. The second reason relates to inyanei even haezer and is not for public discussion.
As for what the girls should be looking for. If you haven’t figured it out yet, the girls are NOT in the drivers seat so long as the numbers are stacked against them.