My Daughters pattern was very similar. Diagnosed at 13, also had slow onset of puberty and growth, and also low iron.
Here's the dilemma in dealing with this disease. After taking Prednisone for the first time, it seemed like a miracle cure for Crohn's. My Daughter started growing normally, and felt very good for about 2 yrs. while only taking Pentasa.
It turns out the disease was actually progressing. She became very ill. Once back under control, she was put on Imuran, (sister/same drug as 6mp)
I wish we would have started Imuran much sooner in hindsight. You want to keep this disease under control as much as possible, to avoid permanent damage.
More recently there is emerging evidence that Pentasa isn't an effective treatment for Crohn's. Many will argue this, and it's a matter of opinion at best.
I would suggest waiting to get your next scope results, and in the meantime, do as much research about 6mp as you can.
6mp/imuran have been around for a long time, and are generally regarded as safe, at the lower doses that Crohn's patients use it at.