The answer is complex and imperfectly understood. In fact, we don't even know why we age. It is taken as a given that we (and every other organism) age, but the fact remains that there isn't an adequate hypothesis about the reason the aging process has evolved to be as it is. However, all that said, we do know some basic facts about how we age, and why it is important to health.
Specifically regarding MSCs, . They offer three generally accepted intrinsic mechanisms by which our MSCs decline: dynamic system malfunction; energy metabolism defects; and DNA damage accumulation. What do these terms mean? "Dynamic system malfunction" is a shorthand way to express that as our cells age, they are less able to transport proteins around internally. Specifically, actin, the main protein that gives all cells structure, is less mobile in older cells. This affects the cells' ability to migrate around their environment and sense external stimuli, among other tasks. Actin is the most abundant protein in the body and is required for many functions, so inhibiting its ability to move has many deleterious effects.
Second, energy metabolism is a vital function in every cell, especially in the brain. Energy is produced by chemical reactions of sugar and oxygen, a reaction that is similar in principle, if not in mechanism, to burning gas or other hydrocarbons. Just as metal discolors when exposed to burning, the organelles that produce energy in the cell, the mitochondria, also become oxidized the more sugar and oxygen they burn. This oxidation makes the mitochondria work less efficiently, and divide less rapidly (mitochondria divide independent of the cell). This means that none of the cell functions that depend on energy inputs (i.e. almost all of them) can proceed as quickly.
We accumulate DNA damage due to reactive chemicals and radiation bombarding our cells. Normally, when DNA is damaged the cell can react and fix the problem. However, there is an error rate in this process not dissimilar to copying a picture in a copy machine. The longer we live, and hence the more DNA damage events occur, the more likely it is that some of our cells don’t fix the problems correctly. Added up over time, this damage can affect genes or gene expression, causing the cells to behave abnormally.
These mechanisms do not represent all the ways that aging effects us, but it is a nice synopsis of what is going on inside the cell (obviously, a lot goes on outside the cell, too). The three processes outlined above share one common feature: time. The longer we wait, the more damage will inevitably accumulate. Freezing our cells effectively stops the aging process of those cells. No biological reactions are known to occur at cryostorage temperature. Therefore, we can be confident that when cells are thawed at any point in the future, whether it be a day or a decade, they will be characteristically similar to the day they were frozen.