While a small baby can generally be taken out with you, there will come a time when you will want or need to leave him in the care of someone else. As he gets older, taking him out, especially at night, will be disruptive to his routine.
Sometimes members of the extended family may be able to take care of the baby on a regular basis. Alternatively, there may be an older sibling who can perform this task. It is difficult to specify at what age a child can be entrusted to look after his or her sibling(s). It depends on a number of factors such as the maturity and personality of the older child, the number and ages of the younger siblings that will be looked after, how long the parents will be gone, and so on. It is unlikely that a pre-adolescent child will have the maturity to be able to take responsibility for younger children on a regular basis.
Often you will need to seek the services of a babysitter.