IMO:
1. I think if they have the mid-level retardation, then I believe they should be allowed to try and live a normal life. As they can still offer towards society and live a nice life.
2. If the case is extreme, and they wouldn't be able to leave a wheelchair/bed for their whole lives. Then I believe it should be up to the parents. If they have the time & money to look after their child, then yes, by all means do it. But if they aren't finically well off, or have a time consuming job, they should be allowed to 'terminate' the child, as it would hugely impact their life for the worse.