First, the TX date is on or before September 1 (state law, not set by district). And K is optional, too. What it says is that a child must be 5 on or before September 1 to start K. If 7 on or before September 1, a child MUST be enrolled in 1st grade (public, private, or home schooled). If age 6 on September 1, a child may be enrolled in either K or 1st grade. If your child has completed K under different guidelines at an earlier age (i.e., started K in a state where they had to be 5 on or before December 1 and the birthday was November 15), they will not be held back.
Redshirting is common. Many people start red shirting as early as May birthdays where I live (in TX). So, if you have a child with a September 1 b-day and they go the first time they are eligible, they WILL be the youngest in the class, but if you hold them, they will most likely not be the oldest. I guarantee they wont be the oldest.
As has been mentioned, you cannot control when you conceive. Ive been trying for over two years now for baby #4 and it just isnt working. It took me two years to conceive baby #2. Baby #1 was first shot out of the gate and baby #3 was a birth control failure. So, I wont worry too much about when to start trying.
Then, every kid is different, and I DONT buy into that boy v. girl thing; I know too many kids that are exceptions. Too many for it to be a rule. DS#1 was ready for K by the time he was 4yo. Most of his friends were (and still are) a year older if boys or are girls. He is very mature for his age. And very, very bright. If he had a September 1 birthday, I most certainly would have sent him.
DS#2 is a whole different story. Hes right on target maturity wise. Ahead in certain areas academically and behind in others. He has a January birthday, so no worries; Im not sure what Id do if his birthday was September 1.
DS#3 is driving me nuts. He IS that September 1 birthday. Hell be turning 3 soon and still barely talks compared to other kids his age. However, when he does talk, it is apparent he is very bright. He knows his letters, numbers, colors, animals, sounds, letter sounds, how to count, and even (as I found out a couple of days ago) how to read some simple words. I DO NOT work on anything with him like this. I have no idea how he gets this; the preschool is a social based preschool and doesnt push any of this either. His maturity level, right now, does seem to be on target for kids who are several months younger. Although, when around older kids, he acts much older than when left alone. I have him behind in preschool (hell be in the older twos class v. the younger threes), but neither myself, DH, last years teacher, or the director are sure if this is the right move. The preschool will be watching him closely and will move him up if necessary. Were thinking he may move up at the semester change. This child is an enigma.
As far as red shirting goes, Ive heard/read/researched mixed reviews on it. I have a neighbor who swore by it. Then she found the older her DS got, the more bored he got. A bored child is just as bad as a child who lacks maturity in the older grades. Except, the bored child is actually looking for and plotting trouble. This started in about 4th grade. And sure enough, my research has yielded that by around 4th or 5th grade most kids catch up on maturity. As for my neighbor, she held her 2nd DS, but DS#3 wasnt ready for K until after she had problems with her oldest she sent her youngest on time. Her oldest is a good kid, she just needs to keep a *constant* eye on him and be really, really strict to keep him on the straight & narrow. All her kids have late summer birthdays.
Then, you have to keep in mind some kids are late bloomers and hit puberty around 14, not good for the kid who is already the youngest in the class. But the reverse is true too. Some kids start puberty as early as 9 or 10. Not good if your kid is the oldest in the class. And really, there isnt any way to predict where your child is going to fall. You can guesstimate based on parents, but each person is an individual.
I have two siblings who were just above the cut off date. My mom sent both. To this day, she regrets sending my sister (who seemed the most ready of the two) and says it was the best thing she has ever done for my brother. What a lot of people perceived as not ready for my brother turned out to be a personality issue that wasnt ever going to change. My sisters issues really came from going to college too early; something that might not have been an issue if my parents had made her go to community college a year or two part time first. My brothers issues didnt leave until he left for college; that is when he came out of his shell. And in college, you dont hang out with your age/grade level; you hang out with those you have things in common with!
Most of what Im reading now says, put kids in if they are academically ready. Holding them back so the appear to be the smartest in the class doesnt make them smarter and harms them in the long run. What researchers are suggesting is to instead find them friends outside of school who are in the same place otherwise with them and keep their extra curricular activities on their otherwise maturity/readiness level. It really does seem to make sense to me, where in school there isnt a lot of social time anyhow (one recess next to lunchboth in 45 minutes).
Id set it aside for now. When your child is at the K threshold, Id evaluate the situation as best as you can then. Coming from someone who has been watching it long term since she had a September 1 baby.

And many kids repeat K without any long-term effects.