The advantage to a will is that you can specify where all of the assets will go in one document. You can name beneficiaries to each of your accounts (usually as a "payable on death"). However, you cannot name a beneficiary of your house, but you can specify in a will where you want your interest in the house to go. Without the will your probate assets (anything that does not pass by way of a beneficiary designation or other joint ownership) will pass under Washington's intestacy laws. Who receives them is a question of your marital and family situation as well as the character of the assets (as either community property or separate property). In other words, without a will you are leaving the determination of the distribution of your estate to whatever unknown conditions might happen to exist at the time of your death.
At what age? The age when you have assets or people you care about and want to protect. From your desciption you have both. Savings and investments may have POD (pay on death) Beneficiaries, but what happens if all your POD Beneficiaries don't survive you? Do you have siblings, nieces, or nephews? You also don't mention how your house is titled, as the previous answer states, a will allows you to build in some redundant safety features in your planning. Also, if you may have children in the future, you can plan for that already now, most states allow you to plan for future children in your will, (not sure Washington permits this, but I would be surprised if it didn't).
My answer typically is that you need a will if you own real property, have minor children, or want assets to be distributed contrary to the intestate succession laws. Probate without a will is a bit more complicated so right there the will easily pays for itself. An additional document might be beneficial for you -- called a Community Property Agreement -- which avoids probate entirely on the death of the first spouse. But you should consult with an attorney before considering such a document.