Have you ever wondered exactly how good you have to be to be in a symphony?I know i have.Is there an age limit?How exactly good are the people in symphonies?How come they dont get payed much is they spent decades on their profession?

If you really want to find out, try taking a few lessons from a symphony player. You'd be amazed.
Symphony players must have exceptional and consistent time, intonation, tone quality, and probably hardest of all, stylistic accuracy. It requires years of study, not just of the instrument, but of history as well. There is a lot more to it than most people realize.

Is there an age limit?and also if they worked so hard for this job why does it pay so low?

No, no age limit that I know of.
Who says symphony jobs pay so low? Major symphony jobs pay very well. Plus, keep in mind, a symphony job is a big foot in the door to teaching jobs, outside freelance work, etc.

Hmmm...Everyone says symphony jobs get paid averagely and they join other bands or groups to make up for it.And when you said take lessons from a symphony player,i dont know how...There is a private lesson teacher at our school for trombones but not quite sure hes in a symphony

Smaller city orchestras don't usually pay that well, but usually that is not a person's main job. Major orchestras are a different story... I believe the base salary in the CSO is $160k right now. You have to basically be THE best to land a job like that though.

I happen to live in Houston which I believe is the 4th biggest city in the United States...I dont know if just because the city is big the symphony is good.

Also...Has anyone ever heard of Jones Hall?

Andre Previn was making one million a year conducting the Los Angeles Philharmonic, and that was two decades ago (not adjusted for inflation).

Repair shops would be a good source to find out who is in the symphony and gives lessons. I play both trombone and cello and when I wanted to take lessons on the cello I asked the local Luthier and he had a list of ten cellists that were in the symphony and gave lessons. The closest one to me is who I called and they turned out to be the best one. The principal chair of the local symphony. Also a Luthier could put you in touch with string players who would know the trombonist. Also you might try the local music shop who sells instruments. That is where I went to find the Luthier's name.

There are trombone lessons listed here. http://houston.craigslist.org/search/lss?query=trombone+lessons
You can contact them and see if they have any connection to the symphony or know how to contact them.

According to the official Houston Symphony web site http://www.houstonsymphony.org/about/musicians.aspx
there is an opening for a bass trombone player