So, I am sure you people are all aware of the fact that level 18 druids have slowed aging, with 10 years equaling 1 physically. If you were to pick a wood elf, which is a perfectly viable race for a druid, and reach level 18 at around 150, you could live for about 5500 more years. That is pretty cool, but not crazy overpowered. However, when I was perusing the dungeon master's guide looking at magic items I saw something that caught my eye. The manual of gainful exercise is a very rare magic item that allows the bearer to study it for 48 hours over a period of 6 days, then increase their strength score and your strength score maximum by 2. But here is the catch. It loses its magic when you do this, but regains it in a century. This seems like a good way to balance it, except for if you live for 5,650 years. If my math is correct, than you could theoretically have a level 18 druid with a strength score of 110. 110! this also applies for the manual of agility and whatnot. So, yeah. Level 30 strength can lift 900 pounds. Just imagine what 110 could do. Jeez.

You may also like the Rod of Security to extend this further or not require Druid.

Most campaigns span less than a year in-game time, in my experience at least. If you give characters millenia to accumulate power when a couple of years already tend to largely suffice to go from level 1 newbness to level 20 pseudo godhood, I'm not sure what you would expect anyway.  

Afraid this doesn't work.  Player's Handbook p.173 says that 30 is the hard limit on ability scores, and the lead rules designer for the game has specifically agreed on Twitter that the various manuals are subject to that limit.

OTOH, if you don't think a level 20 druid with straight 30s and 3000 years of compound interest on his side would be crazily overpowered, I'd think again.  That's one of the reasons my elves are guided by the Prime Directive and stay out of human affairs.  They all have 30 WIS and know it's for the best.

Yeah, moreover who is your millennia year old Druid going to be playing with.  If you set yourself up to outlast the rest of your party, what do you expect everyone to keep rerolling succeeding generations of characters for your to party with?  Multigenerational campaigns are possible, and the Druid and other means of longevity can help foster it, but as Pang pointed out, they're not really all that common so pragmatically your pointing out more a theory flaw than something that will break actual play.

I also want to say some of ability score boosting books and manuals specify that a particular PC can only benefit from them once (for example, in this case, what are you going to relearn the exercise program again, that would imply the prior program some how go lost from you so you would've lost the boost).  I think the 100 year marker is more to discourage book sharing in this regard.  Again, it doesn't appear to be written with this particular manual, but I'd easily house umbrella this under other magic book practices.

Plat has a good point... it makes no difference that the book comes back in 100 years, if the game ends before 100 years go by. Unless everyone is playing elves, no other players would want to advance the time by centuries just to help one player max out his character.

technically, it could work as you stated. If you want stats beyond 30, more power to you. It's your game, do you.

One thing to consider though. In the 5000 years that pass, some ancient wizard council with blessings of the gods didn't die. They are immortal. They've since created items with such magical binding that regardless of strength, they cannot be pushed, pulled, lifted, etc. They've since created horrible evocation magic that doesn't require a saving throw. It just hits and deals death damage. Max HP doesn't matter. A Constitution of 100 doesn't matter. Death saves don't matter. They've since created psychic effects that are so horrifying, no intelligent mind can withstand it. Actually, the more intelligent you are, the more it assaults your mind. This shatters the mind, rendering spells impossible to cast and reducing you to a mumbling fool- but it only affects those with 30 Int and above.

There's always a way around things. Just break more stuff.

it was intended as more of a cool idea, not really something to be tried in game. I just noticed it and thought it was cool. Also, another cool thing you can do is with the enlarge and reduce spell. If you get  10 or so level three wizards to all cast enlarge on a 6 stone, it would be increased like crazy and blow up the earth.