This is from my own experience with family members, so please bear with me.

My Grandmother (dad's mom) didn't get offically diagnosed until she was in her 50s. The progression seemed much faster, but she had issues and flares much of her adult life that the doctors at the time ignored or didn't understand. Being a pathologist, she knew it was neurological - but they couldn't say for sure what it was until later. So in her case, it was the fact they didn't catch it until her symptoms got bad enough.

My mother, who had a very rare and aggressive form of MS, was DX'd at 29 and was totally disabled and bed-ridden by 39. I believe they call it PRMS now.

So like everything else with this disease, I believe it varies case to case. So far even my own path seems to be different. Dx'd at 33, and knock on wood - stable for 3 years.