Is doing a body cleanse or chelation therapy to remove heavy metals in older children and adults with long-standing autism of any real benefit, or has the damage been done and it would be too late to make a difference at this point?

It is never too late until the point of death to remove toxic materials from the body. There will always be a benefit regardless of where and when the intervention is done. Damage is damage and every bit that can be avoided can allow a material gain eventually, with further healing in particular, but one must start somewhere. It is much easier to do the healing and keep that improving over time when the poison has been removed than when it is still present in the body to do its worst. Healing under those circumstances will be fleeting in its benefits.