Most children are dry at night by age 3 and a half. If a child has been dry during the day for 6 months, has sufficient bladder capacity and the parents feel the child is ready then night toilet training can begin.
Start by using waterproof bedding and run a short-term trial without diapers to see if they are wetting at night. If the wetting reduces then continue. If not, then you could introduce an alarm.
Make sure you demonstrate how the alarm works and what it will do when the child wets the bed. The sound may frighten younger children so let them set the alarm off a few times while awake so they get used to it. Reassure them that you will get up at night when the alarm triggers to help them with their night time toilet training.
Many parents of younger children find the DRI Sleeper Eclipse Special Package useful as it includes an alarm for the parent's room which will trigger and wake them at the same time as the alarm in their child's room. This enables quick support for the younger bedwetter.
It is important not to put pressure on a young child and to treat the bedwetting alarm process as routine.