A TF is considered to have life-long expression in a specific cell type if:
Its mean expression level in that cell type in old mice is at least 50% that of the mean expression in young mice, and
The mean expression of the TF in that specific cell type is higher than the average expression of the TF across all 18 cell types, in young mice.
Young mice are ~3 months old, with data from Tabula Muris.
Old mice are ~2 years old, with data from Tabula Muris Senis.