Under normal conditions, the minimum effective level for shifters is 15, while the minimum effective level for druids is 18.
The damage for each age category is given in the following table. (Aside from an anomaly at level 37 and a minimum of 1d10, the damage increases by 2d10 for every 3 levels past 10.) Four more dice are rolled if the caster has at least 10 shifter levels. (Italicized entries are not possible under standard circumstances.)