Last year, Ms Siah Yuet Whey bought a Housing Board flat that is older than she is.

She is 28 years old. It is 44.

This means that she will most likely outlive its lease - which runs out in 55 years. She will be 83 then.

That did not stop her and her husband, 31, from paying more than $700,000 for the three-room unit in Jalan Ma'mor, in Whampoa.

At 861 sq ft, it works out to $854 per sq ft - the third-highest amount paid last year for flats with less than 60 years of lease left. "It is a rare terraced unit in an area with a lot of character, and it does not feel like it is very old at all. We think it is a fair price," said Ms Wong.

Just 40,000 flats picked for Sers since 1995

Out of around a million Housing Board flats in Singapore, only 4 per cent have been selected for the Selective En bloc Redevelopment Scheme (Sers) since it began in 1995. This translates into 40,000 units. They are located on 80 sites. Of these, 73 have been completed - meaning that all residents have moved out, according to data from the HDB. These are typically sites where the land has not been well utilised and has good redevelopment potential, National Development Minister Lawrence Wong said last Friday. There must also be suitable replacement sites available for residents. The Government's financial resources will be considered as well. Under the scheme, the HDB will acquire and demolish old flats. Affected residents are compensated based on market rates. They are also guaranteed a replacement flat in new blocks, with a fresh 99-year lease. Depending on their eligibility, residents also get a fixed sum in the form of a Sers grant - $15,000 for singles and $30,000 for families. Sers residents will be given priority if they are to opt for a new flat in a Build-to-Order or Sale of Balance Flats exercise.

Last Friday, National Development Minister Lawrence Wong, alarmed by a news report on old HDB flats that fetched high prices, sounded a cautionary note about such buying behaviour. Some appeared to have bought those units on the assumption that their flats will benefit from the Selective En bloc Redevelopment Scheme (Sers), he said in a blog post.

This is not so, he said. Only a small minority qualify for Sers, which compensates home owners for their flats and gives them new ones with fresh leases. The rest of the flats will return to the state when their leases expire.

In particular, he advised younger couples to buy a home "that covers you and your spouse to age 95".

Mr Wong's comments have attracted a mix of bewilderment and concern, especially among those who live in old units.

"It wasn't cheap, but I thought the value will keep going up," said IT engineer Andy Zhang, 40, who also paid top dollar for an older home.

Last January, he paid $950,000 for his five-room Bukit Timah flat. It is 43 years old and has just 56 years of lease remaining. This was last year's record for an HDB unit with less than 60 years left on the lease.

To pay for it, Mr Zhang sold a newer three-room flat in Clementi.

Still, he said, he has no regrets. "The age of this flat was not an immediate consideration. I bought this place because my daughter's school is nearby and the location is good."

Flats of a certain vintage are more popular on the resale market due to factors such as location - they tend to be in mature estates - size and amenities in the neighbourhood.

Statistics show they account for a disproportionate share of transactions in the HDB resale market.

Almost half of all resales last year were of flats older than 30 years. This is even though such flats make up only around one-third of the HDB housing stock.

Even flats older than 40 sell well, despite loan restrictions on how much buyers can withdraw from their Central Provident Fund to finance such purchases. They form 11 per cent of transactions from 2014 to last year, even though just 7 per cent of all HDB flats are of that age.

Mr Zhang, for instance, said his resale flat - at 1,346 sq ft - is larger than today's five-room units, an important feature for his family of four.

Ms Siah, a property analyst, is confident she can find a buyer within the next five years or so, due to the rarity of HDB terraced units. There are only 285 left in Singapore.

But generally, she acknowledged, buyers will be more cautious about older flats following Mr Wong's warning. Said the chief executive of property portal Digital Real Estate Assistant: "Location is still the prime factor, not age. But his comments mean that people may be a lot more concerned about the age component now."

ERA Realty key executive officer Eugene Lim said: "It is going to be a lot more difficult to find buyers for older resale HDB flats. Prices for these flats may even take a big hit due to lack of demand. From now on, it is quite likely home buyers will view older 99-year flats differently."

Still, the problem looms far in the future for Mr Zhang, now more concerned about the living conditions for his family.

Asked what his plans are if his home cannot keep its value or if he is unable to find a buyer, he laughed. "That is something to worry about in 40 to 50 years' time. Who knows if I will still be around."