Adds context of Southeast Asia's demographic shift in paragraphs 4-5, background on SEZ in paragraphs 8-9

SINGAPORE, Aug 25 (Reuters) - Thomson Medical Group THOS.SI said on Monday it had launched a more than 18 billion ringgit ($4.3 billion) project to build a private hospital and residential and commercial properties in Malaysia's Johor, in one of Southeast Asia's largest such ventures.

The 26-acre (10.52 hectares) Johor Bay project, within the southern Johor-Singapore Special Economic Zone, will include Thomson Hospital Iskandariah, specialist suites, aged care facilities and a life sciences tower for medical research, the Singapore-listed Southeast Asian healthcare company said in a statement.

The project will also include luxury residences, a five-star hotel, and commercial-lifestyle precincts, it added.

Southeast Asia is experiencing a steady rise in its ageing population, driven by longer life expectancy and declining birth rates.

According to the World Health Organization, the region is expected to see a significant increase in the proportion of people aged 60 and above - from 12.2% in 2024 to 22.9% by 2050.

"The time is right: economic tailwinds, infrastructure momentum, and demographic shifts are converging," Kiat Lim, Thomson Medical's Executive Vice-Chairman, said in the statement.

Johor's Chief Minister Onn Hafiz Ghazi said the state welcomed the investment, describing it as a strategic boost to healthcare access and economic growth.

Malaysia and Singapore announced the agreement for the SEZ in Johor in January, aiming to attract investment and free up the movement of goods and people between the two countries.

A new rapid train link across the border is due to be completed at the end of next year.

($1 = 4.1960 ringgit)

(Reporting by Yantoultra Ngui; Editing by Kate Mayberry)

((Yantoultra.Ngui@thomsonreuters.com;))