Hi and welcome,
Q: " if I do have this, am I going to die young?"
A: The most inportant word in this question is 'if' and even IF you do end up being diagnosed with a neurological condition like MS, the answer would be No, because MS is not that type of medical condition.
Q: "What is the life expectancy for someone that is diagnosed at 18?"
A: It's imposible to say how long anyone's will be, its kind of a how long is a peice of string type of question because you don't have a diagnosis and anything can happen between now and 70 years from now...
Q: "Will I inevitably end up in a wheel chair because I lose my ability to walk?"
A: I went to write no but your question states 'because i lose my ability to walk' which is a leading question, cause IF you lost the abilty to walk getting around in a wheelchair would one of the most common mobility aides.
Q: "Is this going to make stupid or cause mental impairment? "
A: ?? not sure if you realise how incredibly offensive this question is!
Q: "Will I be able to get married and have my own children?"
A: Yes IF you want to get married and can have children
Q: "Will I have to be on anti-seizure medication for the rest of my life? "
A: Its actually really uncommon for MS to cause seizures, so depending on what type of seizures, causation etc etc will determine the best treatment for your specific situation.
Life is a very long time considering you haven't been diagnosed so probably not something you should even be thinking about.
Try to keep an open mind and not assume anything, you honestly may not have MS or require anti seizure medication, once you've gone through the tests you'll have a better idea on what this could be so until you know and have been officially diagnosed, its in your best interest to be open minded.
Hope that helps........JJ
Also does th Lumbar Puncture hurt? I am very very very worried about that!