First, we determine that Isabella will be 10*12= <<10*12=120>>120 months old at age 10, to find a number to compare her age to Antonio's with.
Then we subtract the 18 months from the total of 120 months to find 120-18= <<120-18=102>>102 months, which is the age Isabella is currently.
Since at 102 months old Isabella is twice as old as Antonio, we take 102/2= <<102/2=51>>51 months as Antonio's age.
#### 51