How would a baptist respond to the objection that if a child cannot be baptised, neither can an intellectually disabled adult?

Some Christian denominations reject infant baptism, usually on the grounds that young children cannot profess their faith, or understand the significance of baptism. It logically follows from this that an intellectually disabled adult who can't profess faith or understand baptism should not be baptised, either. This is obviously problematic: life in Christ is meant to be open to all. How might a baptist respond?