After the Age of 7 -- Maturity & Reproduction
Panda families consist of mama bear and baby bear, with papa bear completely out of the picture as soon as mama bear and papa bear have mated, meaning that papa bear is about as useful to mama bear and baby bear, at that point, as is a bicycle to a fish.
Though it would appear that pandas live out their lives in this reduced nuclear family type of structure, they do have occasional social contact with one another via communications, namely via vocalizations (they "talk" back and forth) and via the scent trails that they leave in their respective "territories" (the panda is not considered to be territorial, strictly defined, though, for all intents and purposes, the female with cub is), which scents communicate to other pandas all that other pandas, whether male or female, need to know about them, so pandas do not live in total nuclear-family isolation.
Pandas in the wild reach sexual maturity, on average, at the age of 7, while pandas in captivity, due to their state of enhanced nutrition and general health, reach the age of sexual maturity by, on average, the age of 5. A male can tell by the scents left on trees, shrubs, etc, whether a female is in heat. In addition, the female makes barking calls that alerts any male within a rather extended distance that she is available, drawing him closer to her where he can follow the progress of her state of estrous.
A male will traverse the territory of several females, mating with each of them, and since several males follow the same procedure, it means that a female in heat mates with multiple males, the primary function of this behavior being to ensure that the female will be fertilized (well, the pleasure that is thus spread around probably doesn't do any harm to the broader social relations among pandas in general, eh?).
Pandas mate in the spring, in the months of March to May, with each female being accessible to successive males for only a short period of time, namely, between 2-4 days. But fret not, the area's males are fully apprised of when the period is approaching, for they keep tabs on this development by regularly "reading the tea leaves", as it were, i.e., by analyzing the female's scent as the period of estrous reaches its fruition, and, as indicated, the female will have loudly "telegraphed" her willingness to mate via the special barking sounds that she produces, which barking sounds have but one interpretation. Each male will generally fertilize the female successive times over a brief period, before moving on to the next available female.
The period of pregnancy is quite short, requiring only 3½–5½ months. When the time for the birth begins to near, the female instinctively builds a den, and, if she has a teenager in the household, it is at this point that the teenager will be sent packing, i.e., will be sent off to discover the wide world on its own. Since the panda's young are not prepared to fend for themselves until they are around two years old, the female only conceives, at most, every other year, or when the teenager is roughly two years old.
In the wild, if mother panda gives birth to two infants, only one of them will generally be permitted to survive, since it is extremely difficult for the mother to feed herself and suckle two infants – the sheer logistics of it would be quite daunting, given the time demands that a giant panda infant makes on its mother. Moreover, since the diet of the panda is very non-fatty, it is difficult enough for mother panda to convert the nutrients available to her into enough life-giving milk for one infant.
This is yet another of the limiting realities that a diet of bamboo carries with it. There are many creatures in the wild besides the panda that regulate the number of newborn that will be permitted to survive based on the availability of food once the litter arrives. Though it would be a harsh "Sophie's Choice" for a human to have to make, to the panda – as with the other animals who are faced with such a choice – it is as normal as any other part of their lives, and a survival mechanism surely kicks in that releases the mother from the normal instinct to protect a newborn. Indeed, some species may very well devour the surplus infants, not letting any "windfall" food source go to waste (shudder!).
In captivity, surplus giant panda infants are much luckier – they all get to live, even if the surplus infants have to settle for zoo keepers as surrogate moms.
The life of an adult giant panda is pretty much a routine affair, with males fertilizing the available females in the spring of each year (both male and female pandas in the wild can reproduce more or less their entire lives, i.e., until about the age of 20), otherwise making themselves rather scarce, but naturally foraging for food for about two-thirds of every day, and sleeping, lounging about and poopng the rest of the time. The adult female spends the first part of the birth cycle caring pretty intensively for the infant, and as soon as it is able to fend for itself, or when it is roughly two years old, the cycle begins anew for her. Like the male, she will spend the lion's share of her day foraging for food, the rest distributed over playing with baby (toddler, teenager), sleeping, lounging and pooping.