Had Mike fully read up on the Chronivac, he would have learned that age transformations' interaction with the Space-Time Continuum required that all age transformation must balance out the amount of time used or else risk ripping a hole in the Space-Time Continuum. The Chronivac was designed to automatically prevent such tears in the Space-Time Continuum from forming, and as such, if excess Time were leftover from an age transformation, the Chronivac would find ways of distributing the amount of Time until everything balanced out. It would first try to prompt the user to fix the problem themselves, but after a countdown had expired, it would assign the necessary corrections itself. Since Mike and Jeff had left the Chronivac to relax and work respectively, the timer on the Chronivac ran out and so the machine began analyzing ways to fix the current situation before the Space-Time Continuum would rupture.
First, the remaining 24 years that Mike had taken off his life were given to Jeff. Jeff he Chronivac determined was already fulfilling the role as "Head of Household" as Mike had designated him, so taking up the years that Mike had discarded in the Space-Time Continuum simply made sense. As Jeff mowed the lawn, he found himself sweating profusely from the experience and growing a bit more tired from pushing the lawn mower up and down the hilly lawn he had. Jeff had already aged into his maturity, now he took on signs of age as his age doubled and then some with little worry lines, grey hairs, and a bit of huskiness settled onto his body and build. He now thought of himself as not only the Head of the House, but happily married to Karen--his former mother, now wife. Jeff's mental changes also solidified to accept a new reality that was forming--one where he was seeing himself as the father of Mike. He still remembered his old life as Mike's son, but the new reality was becoming more important to his developing mindset--especially since Mike had set it so that Jeff would be more accepting of his new role, while still aware of the transformation overall.
That however left the 6 years that Jeff had taken from the Space-Time Continuum in order to be aged up to 21 in Mike's original change. There was also the fact that Karen, being 38, was too young to have a 21 year old son, be married to a 45 year old husband and have that past history work out so that they were happily married middle aged couple. The past history would have made it so that Jeff would have been a twenty-three year old man who got a 16 almost 17 year old teenage Karen knocked up. While it was legal--the past history of that situation wouldn't have come without serious repercussions to their life that would have far reaching consequences to the Space-Time Continuum and alter more than just one life. Rather than alter as many lives as needed to make the situation work, the Chronivac opted for the simpler solution of having Mike receive back the six years of youth that had been taken from Jeff in order to age him up to 21 in the first place.
As Mike laid back in the recliner he didn't notice his transformation continue as he grew younger. Muscles shrank, bulk vanished, facial hair withdrew, and even some height began to pull in. Soon, Mike was a fifteen year old lanky boy just having had a major growth spurt which had begun making him look less of a boy, with nothing but peach fuzz on his face. His feet no longer dangled over the edge of the engaged recliner, but instead were an inch shy of the edge now. Mike's own memories swirled. He had a stronger recollection of being Jeff's father and head of the house, but now he was starting to think of the new reality in small ways--thinking of himself as Jeff's son.
Jeff found himself exhausted from only doing half the lawn. He never used to be this exhausted from mowing his lawn, but with a cough, he had to admit he was getting a little old for a push mower. Thankfully though, Mike was starting to get old enough to take up some of the chores around the house at long last. Speaking of which, where was the boy?
Jeff paused in mowing and came inside to look for his son only to find him sprawled out in his recliner watching the game. Here he was busting his old ass out in this god forsaken heat, while his boy was inside loafing off. This clearly had to change, Mike was more than old enough now to start taking up some chores and learning how to be responsible around the house.
And so, to make his point, Jeff grabbed the handle of the recliner and gave it a hard yank which surprised Mike and caused the boy to yelp, his voice cracking as he did.
"You've had enough loafing around for one day. Go out and finish the lawn. I've got it half done, so no whining from you about having to do it all," barked Jeff as though he had been parenting for years.
Mike was caught off guard, wondered if his transformation had gone wrong, and had only to look at Jeff, who his new memories were thinking of as his father to know that, something had indeed not gone as intended.