In the assessment of time since forest disturbance, we define disturbance as the drop of TCC from >=10% to 0%. Use following code or the link here.

var mekong = ee.FeatureCollection('users/biplov/gadm-mekong'),
    tcc = ee.ImageCollection('projects/servir-mekong/yearly_primitives_smoothed/tree_canopy'),
    tch = ee.ImageCollection('projects/servir-mekong/yearly_primitives_smoothed/tree_height');

var tccThreshold = 10; // in percentage
var tchThreshold = 5; // in meter

var years = ee.List.sequence(2000, 2018);

var combineDatasets = function (year) {
  var _tcc = tcc.filterDate(ee.Date.fromYMD(year, 1, 1), ee.Date.fromYMD(year, 12, 31)).first();
  _tcc = _tcc.rename(['tcc']);
  var _tch = tch.filterDate(ee.Date.fromYMD(year, 1, 1), ee.Date.fromYMD(year, 12, 31)).first();
  _tch = _tch.rename(['tch']);
  return _tcc.addBands(_tch);
};

var combinedData = ee.ImageCollection.fromImages(years.map(combineDatasets));

var calculateForestMap = function (image) {
  var _tcc = image.select('tcc').gte(tccThreshold);
  var _tch = image.select('tch').gte(tchThreshold);
  var zeroTCC = image.select('tcc').eq(0).rename(['zeroTCC']);
  return (_tcc.and(_tch).rename(['forest'])).addBands(zeroTCC).copyProperties(image, image.propertyNames());
};

var forestMapCollection = combinedData.map(calculateForestMap);

// This is a function to pass to Iterate().
// As images are computed and added to the list.
var accumulate = function(image, list) {

  var forest = image.select('forest');
  var zeroTCC = image.select('zeroTCC');

  // get(-1) the last image in the image collection
  var previous = ee.Image(ee.List(list).get(-1)).toUint8();

  var added = previous.add(forest).set('system:time_start', image.get('system:time_start'));
  // set the value for a pixel to zero which has TCC dropped to zero
  added = added.where(zeroTCC.eq(1), 0);
  // Return the list
  return ee.List(list).add(added.toUint8());
};

var firstDate = forestMapCollection.first().get('system:time_start');
// The first  image in the list is just 0; keep adding to get the age that meets criteria
var firstImage = ee.Image(0).toUint8().set('system:time_start', firstDate).select([0], ['forest']);
var collection = ee.List([firstImage]);
// Create an ImageCollection of cumulative anomaly images by iterating.
var cumulative =  ee.List(forestMapCollection.iterate(accumulate, collection));
cumulative = cumulative.remove(cumulative.get(0));
var tsd = ee.Image(cumulative.get(-1)).rename(['tsd']);
tsd = tsd.updateMask(tsd.gt(0)).clip(mekong);

Map.addLayer(tsd, {min:1, max:31, palette:'red,orange,yellow,green,darkgreen'}, 'tsd');

Happy Earth Science 🙂