SOUTH PORTLAND — Demand for subsidized apartments owned by the South Portland Housing Authority is usually strong, but it’s downright competitive now in buildings shared by low-income senior citizens and disabled younger adults. Growth in both populations is driving demand for a combined 344 apartments in five properties that were built with funding from the federal Department of Housing and Urban Development, said Michael Hulsey, executive director of the authority. Constructed in the 1970s and early 1980s, buildings such as 425 Broadway and Mill Cove Apartments that have long housed mostly low-income seniors are increasingly housing low-income adults under age 62 who have physical or mental disabilities, as required under federal anti-discrimination laws. The percentage of disabled adults leasing apartments in these properties, including St.