Where epigenetic clocks read biological age from chemical tags on DNA, proteomic aging clocks read it from the levels of thousands of proteins circulating in blood plasma. Proteins are the molecules that actually carry out most of a cell's functions, so protein-level aging signals sit closer to functional biology than DNA methylation does, at the cost of being a newer, less exhaustively validated measurement approach.
One of the more notable developments in this space is organ-specific aging scores: research groups have used plasma proteomics to estimate the biological age of individual organs, heart, kidney, liver, brain, separately from the body as a whole, based on the idea that different organs can age at meaningfully different rates within the same person. That granularity is something DNA methylation clocks generally can't offer as directly.
This category is labeled emerging rather than strong/clinical use deliberately: the underlying science is genuinely promising and published in serious peer-reviewed venues, but proteomic clocks have a much shorter track record than epigenetic clocks and haven't yet accumulated the same volume of independent replication. Worth watching, not yet a settled measurement tool.