If you can remember the season that your heir was born then that will be the season in his 17th year that he will come of age.He will be assigned to the unit that is trained in your western-most province.In other words, if you have a horse dojo in Shinano and a spear dojo in Mino,and both are training troops,your heir will be Yari Samurai.Sometimes you may want to suspend training in that western-most province if you don't want your heir in a specific unit.
Satsuma province with the Shimazu clan is a good example of this.You get a bonus for No-Dachi units in that province and it is about as far west as you can get in Japan.I like No-Dachi units,but not as heirs.That's why it is important to remember the season the heir was born,so you can still train No-Dachi till that season.As Jackson stated,saving each season in his 17th year is a good idea in case you goof.Nothing worse than an Ashiguru heir.
Tito