I admit that welcoming individuals with developmental disabilities into our Church isn't something that has been on my radar, but it should be. A recent article in US Catholic, Real Presence, brought these issues to my attention. Obviously, the Church exists for everyone and should be accessible and welcoming to everyone. My parish does have ramps into the entrance and dedicated handicapped seating, but you need someone to open the door (there is no automatic button). There are also stairs to get to the altar which would prevent some from serving on the altar in any capacity. We definitely have some individuals with developmental disabilities that attend our Masses and I think the parish is welcoming, but obviously there is much more that can be done.
One area that is very important to make accommodations is in religious education. The National Catholic Partnership on Disability offers many resources, including for those who are on the Autism spectrum.
Ultimately, though, the responsibility lays with each one of us to be welcoming to each person who walks into our churches. We may not be able to personally do anything about physical barriers, but we can do everything about our attitude.