It is a bit tricky to answer this question just because deciding when a plant is domesticated is sometimes difficult. There are certain morphological changes in domesticated plants that distinguish them from wild variants, but of course these morphological differences didn't appear over night. This means deciding when the botanical remains we find archaeologically are fully domesticated or not is difficult, with many transitional forms. I think the better question to ask, and more to the point of your question, is not when these strains of crops became fully domestic but when they were first widely cultivated as part of early agricultural practices.

To answer your question more directly, lentils are the oldest domesticated pulses, but we have evidence that two varieties of wheat (einkorn and emmerkorn), as well as barley are the oldest cultivated crops, though not by very long. In reality though, there are some problems with the evidence that mean lentils (along with a few other pulses) are one of the earliest crops alongside wheat and barley as a complete agricultural package.

The earliest evidence for cultivation of domesticated varieties of these two wheats alongside barley comes from the Pre Pottery Neolithic B (the PPNB for hip archaeologists) around 10,500-10,100 BP (8,550 - 8,150 BC). The evidence comes from a string of villages in the Levant, southeast Anatolia, and parts of eastern Mesopotamia (i.e. the Fertile Crescent). In contrast, the earliest lentils come slightly later at 10,100-9,700 BP (8,150 - 7,750 BC), with other pulses (like peas) following slightly after the lentils.

All that said, in all likelihood lentils (and other pulses) where domesticated contemporaneously with wheat and barley. The morphological changes we look for as evidence of domesticated strains of a crop rather than wild strains are very pronounced in wheat and barley, making it much easier to identify clearly domesticated varieties. On the other hand, the morphological differences between wild pulses and domesticated pulses are much more subtle and so we are probably underestimating their earliest domestication due to identification issues. There are probably some issues with preservation as well - wheat and barley tend to preserve better in the archaeological record than do the remains of pulses.

Source: Zohary, Daniel, Maria Hopf, and Ehud Weiss 2012 Domestication of Plants in the Old World. Oxford University Press.