Is it ok to sleep on a wool mattress if i was allergic to wool as a young child?

Possibly: If you no longer demonstrate symptoms to wool with recent prolonged exposures you may be able to tolerate that mattress. You should discuss this in detail with your local allergy/immunology specialist.

Try before you buy: Most people who have wool allergy have itching with contact. Patients with eczema can be especially sensitive to the itchy touch of wool. Nasal allergy & asthma symptoms are very unusual with wool. So if in the past wool has made you itch it's likely to do so again. Try wearing a wool sweater. You'll find out soon enough if you are still allergic to wool.