As Hol said, you aren't too young for treatment. It is true that younger patients do sometimes need further surgeries (as the disease can develop/become more obvious as we age), but that is no reason not to treat the problem now.
Do you know if they excised/removed any of the disease during your lap, or was it diagnostic only? As Hol said, it's probably worth going back to your Gp for another referral. it's also worth checking if there is a BSGE specialist centre in your area, as they are generally more skilled, and thanks to Patient Choice (if you are in England) you can request a referral directly to them with your GP.
Regarding the mirena (and other hormone based meds), these options can sometimes help reduce symptoms in some people, but they don't get rid of the endometriosis, and in some cases people continue to have pain and side effects. I've tried mirena 3 times myself, and always ended up getting it removed (after at least 4 months to give it a chance to settle in and after a year at the longest), so unfortunately in some cases these options just don't work as well as we hope. One two occasions it was awful right from the start, and the pain and bleeding never subsided. One one occasion I did get a few good months on it.. but about 7 months in my bleeding and pain really ramped up and I ended up getting it removed after struggling on for a few more months.
Obviously they do sometimes help, and I have spoken to a few people with endometriosis who think they are brilliant.. but others like me have had issues.. so it's not unheard of, and if you are struggling you have the choice to get it removed.
Personally the side effects (awful central cramping, bleeding, spotting and general discomfort) stopped almost straight away after removal. Obviously though it's an individual choice, and you may be getting some relief from it, but I just wanted you to know it's not a guaranteed fix, so don't feel you have to stick with it if you are struggling.