Re older second marriages: in some ways I am in one of those, and in some ways I am not. That is, it was a first marriage for me, and only in my forties and not my fifties, but for him it was a second marriage with children from the first. I was not bringing any children into the mix, nor would we conceive any together.
The concept of "what to do with our assets" is only relevant for couples who both come into the marriage with a lot of money, or at least one of them does. A prenuptial agreement generally comes with the idea "we may divorce someday." I would thus have not agreed to one, because I do not have that understanding of marriage, nor would I have married a man who did--in other words, there can be other reasons to refuse a pre-nup than deviousness. It is wise for any couple to do estate planning, and trusts can take care of who inherits what. I know a couple who married in their seventies, each with a few million in assets, and they arranged to hold their finances separately for the sake of their respective children. In their case that probably made sense, and certainly none of their children stood to suffer from such a deal.
The thing about second marriages is that they are every bit as much true marriage as the first one. If the parties are in their forties or older, the second marriage is less likely to conceive children--but it may last longer than the first marriage, if they themselves live into their seventies or eighties. Depending on how old the children are, the wife may be more free to hold a full-time job than she would have if they were raising children together--but she may well have used up most of her prime earning years raising her own children, or raising his children or theirs together. In other words, unless they are independently wealthy, they are likely to use most of their assets for living, and a man caring for his wife is a higher life priority than how much he leaves for his children. In our case things are a little simpler, since I have no children and his children are my heirs. (If he were to die and I remarry, at that point we would need to provide for the children in both families.) In our case, I owned a house that had only a few years left on the mortgage (it would have been paid off by now, had I remained single), and those assets have been brought into the family as they would have been in a first marriage for both parties, and my husband does not wish to have me hold a full-time job outside the home--so making sure that I am provided for is more necessary than anything else in our estate planning, just as would be the case in a first marriage for both. At the same time, our combined assets are more than either had together, and we have a little more ability to help young-adult children now than he would have had alone as a widower, so the net result is better for everyone, I think.
One also never knows how much the final years will cost. I know someone who did little saving in decades of working a good, professional job, with the assumption that most of the assets of the parents would take the place of a savings account. But the parents' assets ended up going into nursing care for the parent with Alzheimer's. It would be unwise, in other words, to tie up everything one had coming into marriage for this set of children or that, without taking into account that the household itself might need the money, or the widow might need the money. Protection against a greedy spouse who divorces and takes off with the assets is another situation entirely--and the best protection for that is somehow figuring it out before marriage, and not marrying the person. But don't protect against the possible shyster in such a way that the committed spouse gets hurt.
Marriage--what a wonderful image of Christ's love for His bride!