We use Spitzer IRAC 3.6-8.0μm photometry of late-type T dwarfs to investigate various trends which can aid the planning and interpretation of infrared (IR) surveys for the coldest T or Y dwarfs. Brown dwarfs with effective temperature (Teff)<700K emit>50% of their flux at λ>3μm, and the ratio of the mid-IR to the near-IR flux becomes very sensitive to Teff. The color H - [4.5] is a good indicator of Teff with a weak dependence on metallicity ([m/H]) and gravity (g) while H -K and [4.5] - [5.8] are sensitive to [m/H] and g. Thus Teff and g can be constrained and mass and age can then be determined from evolutionary models. There are 12 dwarfs known with H - [4.5] > 3.0 and 500 TeffK 800, which we examine in detail. The ages of these dwarfs range from very young (0.1-1.0Gyr) to old (3-12Gyr). The mass range is possibly as low as 5 MJup to 70 MJup, and [m/H] also spans a large range of ∼ -0.3 to ∼ +0.3. The T8-T9 dwarfs found so far in the UKIRT IR Deep Sky Survey are unexpectedly young and low-mass. Extensions to the warm Spitzer and WISE space missions are needed to obtain mid-IR data for cold brown dwarfs, and to discover more of these rare objects.