I've been wondering about this for a few days. The game's potential formula states that the potential of two monsters' offspring depends on the parents' potential and the total AP between them. However, it appears that combining monsters in their prime yields excellent results, at least with respect to starting stats.

I'm not sure if it's ever been made crystal clear here; is it better to combine two monsters with high potential early in their life, or during their twilight once they've accumulated a large amount of AP?

It's hard to say. If the Perfect Guide is to be believed, then Age was a factor in MFDS1 but it is no longer a factor in MFDS2.

I have been waiting until they get a retirement notice and still get Twin Galaxy thru Universe level potential.

I may have spoken too soon. I am not seeing Age as a factor in MFDS1 either.

Looking at the Potential Formula directly (Perfect Guide page 40) it appears to say:

New Monster's Potential = [combo monsters' potential class] + [combo monsters' AP bonus]

The first factor appears to be a weighted average of the potential class of both combo monsters. It appears that the monster that contributes the main breed factors higher than the monster that contributes the sub breed.

The second factor appears to be a total of all the action points between both combo monsters. It doesn't appear to state exactly how that bonus is comprised or how many AP it takes to affect the result. It merely states to get it as high as you can.


So, if this is to be believed, then it seems that it would be best to wait until old age.

I visually dissected the whole section on Combining to try to find any Kanji that related to Age (specifically - Age, Prime, Mature, Old Age) and can find none.

I went back to look at the over all Combo formula I translated here -
Monster Rancher Metropolis: Monster Rancher DS Archive (DS): Japanese DS Games Archive: Monster Farm DS 2: Misc. QA (post all new topics here, well make more sections later) : Combo Formula

After going through it again, I stand by it. However, I did figure out what the mention of D Rank meant - you get a story event about combining after you have won the D Rank Official Cup.

Also, my note about the weighted average above is actually supposed to be the same as in the Japanese Combo Formula thread. The best result for the average is if the 2 source monsters each contribute their Main breed to the new monster. Here is the way it appears to break down (this is my own chart, not a guide chart) if we have 2 monsters (M1 and M2) and we'll say that M1 is a better monster than M2 (stat-wise) -

Main Source	Sub Source	Result
M1 Main	M2 Main	High
M1 Main	M2 Sub	Medium
M2 Main	M1 Sub	Low

The best results are obtained when the better of the two monsters provides the new main breed and the second monster provides its main breed as the new sub.

I've been experiencing good results combining one old champion monster with one monster which has just hit it's prime.

The game did say that monsters are best to combine in their prime, and somebody here repeated that to me.

Well, I guess that settles it, then. Thanks, Thorr!