How bad is a Bilirubin level of 12% for a 24 days old boy?
What are the side effect? Can this cause any lasting damage?
Do you mean 12% or 12mg/dL?
There are two types of jaundice physiological and pathological.

Physiological jaundice
This results from increased red blood cells breakdown and immature liver function.
It presents at 2 or 3 days old, begins to disappear towards the end of the first week and has resolved completely by day 10.
The bilirubin level does not usually rise above 12mg/dL and the baby remains well – sucking, no fever, no abnormal movement, nothing else than the jaundice.

Jaundice in the first 24 hour of life
Total Serum Bilirubin concentration greater than 12mg/dl (204umol/L) at any point in time
Serum Bilirubin concentration increasing by more than 0.5mg/dl per hour or 5mg/dl per day
Conjugated bilirubin of > 2mg/dl (34umol/L)
Jaundice persisting for more than two weeks
Haemolytic jaundice where the baby’s blood level is also dropping (anaemia).