I was diagnosed at 18 months old. I'm now 43. I had pretty severe symptoms until I was five when I went into remission until I was 11 and flared badly. I had a mixture of treatments before 5:
aspirin
fluid removal and steroid injections
steroid tablets
Hydrotherapy and Physio
Splint wear at night to straighten my legs
Between five and eleven I had a pretty normal childhood, with some restriction - no jumping off of things, no sports which could 'jar' my knees.
Between 11-16 I was pretty bad. Fluid removal and steroid injection every few weeks and I was also on Brufen and ibuprofen. Restrictions on sports at school and unable to walk far. Had regular eye checks (about twice a year) at the hospital - all fine.
Was in remission from 16-21. Flared but not as bad as teen years. Was on Naproxen and Sulfasazine. Kept me ticking over and I got my self off all medication by the time I was 30.
Flared again after giving birth to my child when I was 34. Probably my worst flare yet - walking with sticks, difficult to carry my baby around. Had steroid injections and back on Sulfasalazine. I am still on Sulfasalazine now and it keeps me ticking over.
i was told at age 15 that I would probably be In a wheelchair by age 40 - I'm not!
it's unpredictable but you can do things to help, such as gentle exercise (swimming is perfect), supplements (cod liver oil etc), knowing your limits etc
My husband also has a chronic illness, diagnosed later in life and he struggles more than me because he remembers 'normality', whereas I don't. I have always treated my JIA like a friend - I am respectful to it, and it respects me. I know not to walk too far, or do things that are hard on my joints. The fatigue is the worst thing - I work full time (I was once told I'd never be able to do that) but that means I have to go to bed early and get rest when needed. I have to say no to things like social events if I know I have a busy time at work.
It's not ideal but it can be managed and I think that in the next few years advancements in medicine will see much better treatments for JIA.
Please try not to worry - I know easier said than done but it won't change anything. Invest time in looking at things to help (like the swimming etc).