About as much as telling your kid to not drink beer while you are doing it yourself.
The phrase "drink beer" could mean anything from 2 beers on a sunny weekend to 6-pack every night. They are not comparable.
Why would I tell my son not to drink beer? He can do so as soon as it’s relatively safe to
Aside from the physical/chemical damage alcohol does to developing brains, alcohol addiction is bad for adults too. We just concede that after childhood you can make your own choice to ruin your life.
Phone addiction is harmful to everyone at all age groups. It's not really the individuals to blame through. The tech companies have broken human psychology and developed something more addictive than drugs.