It wasn't necessarily "age in bond" stuff. More likely "age in fixed income".All that 'age in bonds' stuff was born in an age much different than we are in today...when bonds delivered real returns and the equity risk premium was just 1-2% or so. Attempts to tweak it with 110-age in equities, or in later years with 120-age, was/is just a lazy, lame attempt to justify slightly higher equity percentages due to the widening of the gap between bonds and equity risk premiums.
As I tried to point out above, it could still be a valid rule. Sources of fixed income other than those from investments should be considered before deciding on allocation ratios. Everyone seems to be ignoring this, although it is a key factor. Some Reading.