Sorry to say I agree with most if what your manager said.
First, although you stated you were not available to work weekends, once he scheduled you the first time to work a weekend you should have given him a gentle reminder that you cannot work weekends. Since this was not written it is quite possible he forgot (managers have alot of juggling so i can easily see this could be the case) and needs and\or work situations change so he might have needed someone to work on the weekend. You not reminding him could have put your company and whoever else is working on the weekend in a tough situation. You basically ignored this and went about what you wanted without any regard for anyone else. It was irresponsible.
There is also nothing wrong with him pointing out that your sakes are the poorest. He should be doing that as part of his job is to relay job performance.
What he should NOT do is state anything about age. Your age and those of others do not matter...he should have kept to the facts...that you did not communicate with him that you were unable to come in on the weekend and that your sales were below expectations.
On the flip side did he verbally discuss with you not coming in on weekends before he wrote you up? Ideally a manager gives you a verbal warning and then a written one if you do not comply with the request. He should also be very clear on your expectations going forward ... For instance your must be x or higher within 3 weeks or this will happen.
One thing I can suggest after you digest this is to take the initiative and make a meeting with him. Make note ofwhat he said that distressed you. Just note the facts ... Then let him know it is inappropriate to reference a person age .. air that out but be professional. If you feel he was not clear on your job responsibilities or expectations, ask him to outline these as you want to focus on improving. Make him do his job which is to be clear on what you need to do to meet and then exceed expectations. Also ask him what he would prefer you to do if you are unable to cone in on certain days? Do you need to put something in writing, should you call, text, etc.
For you listen to him as far as steps you need to take to improve. Focus on that not anyone else or your age. No thing to note a younger person dies not have your experience not just on the job but in life experiences so you have value that someone much younger does not.
I manage people and I have had to hold hard conversations and let people go that were not performing ... I am also "seasoned" ...a term I know is used within the recruiting and hiring world to mean older so they don't get into trouble. I would make nite though of any reference to age he has made as it is inappropriate and may cross the line legally just in case you need it.