You do see 18-year old seniors on the football field in their senior season. It's not unusual, but I wouldn't say it's the norm.
One example: I was told that when STM won its first state championship in 2017 that entering the season they had one 18-year old senior on the roster along with two seniors that were still 16. The administrator who gave me that info said that the Catholic schools in south Louisiana had similar numbers. Even John Curtis, in the past decade, has had far fewer 18 and 19-year olds than in previous decades.
Also, Nic Brown was on Fox's afternoon drive show in Ruston a few weeks ago. He indicated the LHSAA was going back to allowing the hold back decision to be made in the 8th grade. For more than a decade the decision to hold a student back in junior high had to be made in either the 5th or 6th grade. That dramatically curtailed the hold backs at West Monroe and John Curtis.