I keep re irritating my lower back in golf.

Try strengthing your lower back and develop a routine for stretching not only your lower back but your body everyday, not just before a round of golf.

As far as the exercises for strengthing your back are concerned, a good list can be found at: http://www.bodybuilding.com/exercises/finder/lookup/filter/muscle/id/5/muscle/lower-back

I would add, do a lot of core and abdominal exercises.  The stronger your abs and core is, the stronger your back will be.  At 15, you can really strengthen your abs quickly.  You can do ab exercises, like crunches and leg lifts, every day.  It will make a big difference.

Welcome to my world. I have dealt with this since your age.

I have severe pressure on my sciatic nerve.

There are easy fixes ( such as sleeping with your feet raised ) that help - but walking for an hour a day has been the best thing for me.

Strengthening your butt cheeks will seriously help

How active are you as a 15 year old? Are you a person who's played sports alot, or do you spend your time inside watching TV and playing video games. 

Being young now a days doesn't mean anything with regards to limberness and health. When more and more kids are just staying inside, rather than being active, the lack of adequate muscle strength is alarming. 

The army showed that even new recruits couldn't even do the basic level of strength, while alot more people could decades ago. 

As for back pain, its usually caused by lack of strength somewere else, because your back is not taking the stress for a weakness somewere else. Sciatica is also caused by this, were you have lack of strength and it causes pressure on the nerve. 

There are some good websites for exercises, and good books out there as well. I would focus on the following muscles groups,

Hip Flexors

Hip abductors

Hip Adductors

Abdominals

Gluts

Hamstrings

Quads

Yikes!  They make 15 years olds bigger now than when I was a kid!  You should do an alternate work out to compliment golf.  Core workouts will really help and then shouldn't impact your hip. Check with your doctor first though.  Also, as suggest above, a good flexibility routine with help too.

Hip problems could definitely cause back pain. You'd be shocked at what causes back pain, usually isn't anything to do with the back. My mom's one leg is like quarter of an inch shorter than the other, she wears a foot insert now in her shoe, and it took away her back pain. 

Is it possible for you to have some sort of old injury which hasn't been addressed properly or completely?

Just my 2 cents:

You are really young to have low back pain of any kind. Sure it's possible, but at your age 'pain' is telling you something and should be taken serious (not saying you are not). Yes, strengthening exercises can help but until the structures that have been affected (injured) are healed you run the risk of re-injury and complicating you current issue.

oh and leg length discrepancy caused by various factors (mechanical or anatomical) can cause chronic low back pain.
 

I am new to the forum, but am 43 and have lifted weights for some time. You obviously have a long life ahead of you, and you like to compete. Fitness will always help you overcome many things. You seem to have a great attitude and with proper training you will be a very good golfer.