"Regress!" You shout as you cast the spell of age reversal.
This spell you're sure could come be quite useful in a fight. If you were to regress an opponent in age to a newborn, they'd be helpless, and probaly unable to use the book.
Of course, you don't want to render yourself hopeless, you decided to only use the spell in moderation to find out exactly how young you would be before you had trouble casting the spells.
After the first puff of smoke, you find yourself now to be a teenager of fifteen again. Your clothes are now somewhat baggy, and you've lost quite a bit of weight. However, you of course have no trouble casting spells at this age.
"Regress!" You shout, and another cloud of smoke covers you.
This time, you're a ten year old child. Your clothing is quite oversized and nearly falling off. Realizing it's going to happen at some point during practice, you take your clothes off, and continue your practice naked.
"Regress!" You shout again.
This time, you come out as a five year old. You remember the last time you used the spellbook to make you this young. You were able to change yourself back then easily, so this probaly wasn't going to help much in a fight.
"Regress!" You shout in your voice which grows ever more high pitched.
Now you're three years old. You feel a bit ackward, and the book is to heavy for you to actually hold at this point.
"Alright, so if I turn them into a toddler or younger, I should be fine." You think to yourself.
You feel a bit tired now, but you've only practiced one spell.