Not a fan of fussion. Like Derek said, it causes a domino affect and should only be used as an extremely last resort. I was diagnosed with
Pars Defect L4/5 by one of the top neurosurgeons in Philly and he basically told me stay stretched and stay in shape. He stated the only real way to fix it is with fussion and once fused together, I'm at greater risk of ruptured/herniated discs or further issues up my spine.
That said, considering your age, if fussion is recommended, get a 2nd and 3rd opinion. True, some people may not experience limited mobility while others do. Quality of life question I guess... is it something that can be tolerated with specific stretches every day as well as a strong core or is it a must to have surgery again? Also need to factor in recovery time which I'm sure you know about. I just remember seeing my dad after his back surgery and thinking to myself I never ever want to have to go through that.
Hope everything works out for you.