Certainly helminths can infect people of any age.  And if you have known case of soil infested helminths in a very young child, the child should be treated.  The recommendation that routine treatment in all malnourished children not be given is the result of two factors.  1) Younger children might not  metabolize the drug as quickly, this is simply not known. the drug has not been extensively tested in infants   2) people who do not walk are much less likely to have these parasitic infections.