Unfortunate data layout, but here's one way: in E14:

=IF(B14="F",IF(COUNTIF(D14,INDEX(Severely_Wasted,M ATCH(C14,Month,0))),"Severely wasted",IF(COUNTIF(D14,INDEX(Obese,MATCH(C14,Month ,0))),"Obese",CHOOSE(ROUND(MATCH(D13,INDEX(tBMIGir ls[[From]:[To3]],MATCH(C14,Month,0),0))/2,0),"Wasted","Normal","Overweight"))),IF(COUNTIF( D14,INDEX(tBMIBoys[[Severely Wasted if BMI is ]],MATCH(C14,tBMIBoys[Age in Months],0))),"Severely wasted",IF(COUNTIF(D14,INDEX(tBMIBoys[[Obese ]],MATCH(C14,tBMIBoys[Age in Months],0))),"Obese",CHOOSE(ROUND(MATCH(D14,INDEX(tBMIBoy s[[From]:[To3]],MATCH(C14,tBMIBoys[Age in Months],0),0))/2,0),"Wasted","Normal","Overweight"))))

and copy down.

Thank you Debaser. It took me sometime to reply because when I copied the formula and edited it to fit into my actual data, a lot of N/A came out, caused by children below 72 months of age. I did not notice that there is a separate growth table for infants to toddlers. I will try adding another IFs in the formula that you gave. I hope I'll get it right.