Note at first I understood your question as 'making multiple regressions with one variable' this gives rise to part 1 in which I explain the effect of an interaction term. In the image of part one the left image relates to doing six different simple regressions (a different one for each single age class, resulting in six lines with different slope).
But in hindsight it seems like your question is more relating to 'two simple regressions versus one multiple regression'. While the interaction effect might play a role there as well (because single simple regression does not allow you to include the interaction term while multiple regression does) the effects that are more commonly relating to it (the correlation between the regressors) are described in part 2 and 3.
