Thank you for the information and your question. There would be no way to give you a conclusive answer about the outcome of any claim of discrimination either based on age or disability, but I can discuss the law in general and recommend that you sit down with a local employment law attorney who handles discrimination cases on behalf of employees and who can gather more information from you and other sources, including your former employer, than I can by way of this forum.
First, I will talk about your deadlines, because those are the most pressing at this time. For a complaint under State law, Texas has a 180 statute of limitations within which time you must file a complaint with the TWC in order to preserve your State claim. So, it appears that period may have already past if I do the math correctly. The time starts running from the date of the last alleged discriminatory act. Under Federal law though as long as the State has a laws against the type of discrimination, and the Federal Government does as well, which is the case here, there is a 300 day SOL within which to file a complaint with the EEOC to preserve your claim. You are knocking up against that deadline based on your facts, so you would want to take action very soon if you are going to.
As for the basis for your complaint. Age discrimination under the Federal ADEA is extremely difficult to establish. The law is that a persons age (over 40) must have been the sole reason for their termination. In other words, the employer could have had no other reason at all, including performance or a legitimate economic/reorganization reason. Again though, I cannot guess what the situation is in your case, but can only discuss the law.
In terms of disability discrimination, that basis is somewhat easier to establish as the law only requires that a persons disability, or perceived disability, must have only been a reason for their termination. You would make the allegations, and the employer then has to rebut the allegations with facts that support that your disability wasn't a reason. Again though performance or a legitimate reorganization, as long as they can show that they would have taken the same action with someone who was not disabled, or perceived to be disabled, would be typical defenses.
All that said, we are back to me not really having enough information, including from your employer to draw any conclusions. However, if you signed a severance agreement, it is very likely that you also signed a legal waiver not to file suit for any alleged wrongdoing on the part of your employer. If so, then you can't file suit, whether they discriminated or not. You could still file your complaint with the EEOC, but unless they could get the employer to admit wrongdoing and offer a settlement, it would end there since, again, you would not be able to file suit under the severance agreement.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the ratings box above, I will receive credit for assisting you today. Thank you